I updated with CVS again, on the same system, and I still get a segfault when accessing index.php, but a different one:

[root@total php4]# gdb /usr/local/apache/bin/httpd  
GNU gdb 4.18
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux"...r
u(gdb) run -X
Starting program: /usr/local/apache/bin/httpd -X
[Mon Aug 14 12:25:21 2000] [warn] NameVirtualHost 212.25.240.41:80 has no VirtualHosts

Program received signal SIGSEGV, Segmentation fault.
0x400c2a93 in strncpy (s1=0x81c9788 "active_sessions", s2=0x825e79c "", n=4294967284)
    at ../sysdeps/generic/strncpy.c:82
82      ../sysdeps/generic/strncpy.c: No such file or directory.
(gdb) bt
#0  0x400c2a93 in strncpy (s1=0x81c9788 "active_sessions", s2=0x825e79c "", n=4294967284)
    at ../sysdeps/generic/strncpy.c:82
#1  0x406cb624 in ?? () from /usr/local/apache/libexec/libphp4.so
#2  0x406cd458 in ?? () from /usr/local/apache/libexec/libphp4.so
#3  0x4065c1d6 in ?? () from /usr/local/apache/libexec/libphp4.so
#4  0x4065c427 in ?? () from /usr/local/apache/libexec/libphp4.so
#5  0x4065c427 in ?? () from /usr/local/apache/libexec/libphp4.so
#6  0x4065c427 in ?? () from /usr/local/apache/libexec/libphp4.so
#7  0x4065c427 in ?? () from /usr/local/apache/libexec/libphp4.so
#8  0x4066886e in ?? () from /usr/local/apache/libexec/libphp4.so
#9  0x40677c21 in ?? () from /usr/local/apache/libexec/libphp4.so
#10 0x406748d9 in ?? () from /usr/local/apache/libexec/libphp4.so
#11 0x406751b6 in ?? () from /usr/local/apache/libexec/libphp4.so
#12 0x406751ec in ?? () from /usr/local/apache/libexec/libphp4.so
#13 0x806df73 in ap_invoke_handler ()
#14 0x8081919 in process_request_internal ()
#15 0x8081d48 in ap_internal_redirect ()
#16 0x806163d in handle_dir ()
#17 0x806df73 in ap_invoke_handler ()
#18 0x8081919 in process_request_internal ()
#19 0x808197c in ap_process_request ()
#20 0x807918e in child_main ()
#21 0x807933c in make_child ()
#22 0x8079499 in startup_children ()
#23 0x8079ac6 in standalone_main ()
#24 0x807a263 in main ()
#25 0x4007d1eb in __libc_start_main (main=0x8079f0c <main>, argc=2, argv=0xbffffce4, 
    init=0x804f614 <_init>, fini=0x80b03cc <_fini>, rtld_fini=0x4000a610 <_dl_fini>, 
    stack_end=0xbffffcdc) at ../sysdeps/generic/libc-start.c:90

Any further information needed to help track this one down?

Could you please enable debugging and then make a backtrace?

I used this script to regenerate and build:

#!/bin/sh
for i in "Zend" "TSRM"; do
  cd $i;cvs update -Pd;cd ..
done
cvs update -Pd
./buildconf
make clean
./configure --enable-debug --with-apxs=/usr/local/apache/bin/apxs --with-mysql=/usr/local --with-mcal --with-imap --with-gettext --enable-versioning --with-gd && make 

I get exactly the same backtrace :(  Shown below in case its different ...

GNU gdb 4.18
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux"...
(gdb) run -X
Starting program: /usr/local/apache/bin/httpd -X
[Mon Aug 14 16:55:38 2000] [warn] NameVirtualHost 212.25.240.41:80 has no VirtualHosts

Program received signal SIGSEGV, Segmentation fault.
0x400c2a93 in strncpy (s1=0x82dfb28 "active_sessions", s2=0x827999c "active_sessions", 
    n=4294967284) at ../sysdeps/generic/strncpy.c:82
82      ../sysdeps/generic/strncpy.c: No such file or directory.
(gdb) bt
#0  0x400c2a93 in strncpy (s1=0x82dfb28 "active_sessions", s2=0x827999c "active_sessions", 
    n=4294967284) at ../sysdeps/generic/strncpy.c:82
#1  0x406cb3c4 in ?? () from /usr/local/apache/libexec/libphp4.so
#2  0x406cbe96 in ?? () from /usr/local/apache/libexec/libphp4.so
#3  0x406cc515 in ?? () from /usr/local/apache/libexec/libphp4.so
#4  0x4065c854 in ?? () from /usr/local/apache/libexec/libphp4.so
#5  0x4065ca6c in ?? () from /usr/local/apache/libexec/libphp4.so
#6  0x4065ca6c in ?? () from /usr/local/apache/libexec/libphp4.so
#7  0x4065ca6c in ?? () from /usr/local/apache/libexec/libphp4.so
#8  0x4065ca6c in ?? () from /usr/local/apache/libexec/libphp4.so
#9  0x4066932e in ?? () from /usr/local/apache/libexec/libphp4.so
#10 0x40679041 in ?? () from /usr/local/apache/libexec/libphp4.so
#11 0x40675c79 in ?? () from /usr/local/apache/libexec/libphp4.so
#12 0x40676596 in ?? () from /usr/local/apache/libexec/libphp4.so
#13 0x406765cc in ?? () from /usr/local/apache/libexec/libphp4.so
#14 0x806df73 in ap_invoke_handler ()
#15 0x8081919 in process_request_internal ()
#16 0x8081d48 in ap_internal_redirect ()
#17 0x806163d in handle_dir ()
#18 0x806df73 in ap_invoke_handler ()
#19 0x8081919 in process_request_internal ()
#20 0x808197c in ap_process_request ()
#21 0x807918e in child_main ()
#22 0x807933c in make_child ()
#23 0x8079499 in startup_children ()
#24 0x8079ac6 in standalone_main ()
#25 0x807a263 in main ()
#26 0x4007d1eb in __libc_start_main (main=0x8079f0c <main>, argc=2, argv=0xbffffce4, 
    init=0x804f614 <_init>, fini=0x80b03cc <_fini>, rtld_fini=0x4000a610 <_dl_fini>, 
    stack_end=0xbffffcdc) at ../sysdeps/generic/libc-start.c:90
(gdb) 

I've updated on the same system and script to latest CVS, and things compile and run now.

However, the include_path behaviour seems to have changed, and the '.' in my include_path is no longer being recognized - was this intended?  I can no longer include files as 'templates/blah.inc' which worked in a previous 4.0.2-dev checkout, and all earlier versions of PHP.

No, "." should work and it works for me. If you have reproducing case (i.e., include-path setup that doesn't work in lab conditions) please open a new bug report for that.