I forgot to mention the most important part:

if zlib is not available!

I met the same failure.

Environment
---------------------------------
php 4.3.2  without zlib support
$Id: go-pear,v 1.67 2004/09/09 06:25:17 cellog Exp $

I got these messages.
---------------------------------
(left out)
The following PEAR packages are bundled with PHP: DB, Net_Socket, Net_SMTP,
Mail, XML_Parser, PHPUnit.
Would you like to install these as well? [Y/n] : n

Loading zlib: failed
Downloading uncompressed packages
Downloading package: PEAR-stable......ok
Downloading package: Archive_Tar-stable....ok
Downloading package: Console_Getopt-stable....ok
Downloading package: XML_RPC-stable....ok
Bootstrapping: PEAR...................(remote) ok
Bootstrapping: Archive_Tar............(remote) ok
Bootstrapping: Console_Getopt.........(remote) ok
Extracting installer..................Unsupported compression type ''
Supported types are 'gz' and 'bz2'.
---------------------------------

How about this patch?
At least, it worked for me.
---------------------------------
--- index.html.original Thu Sep  9 22:17:12 2004
+++ index.html  Sat Sep 11 01:10:29 2004
@@ -649,7 +649,7 @@
 $ext = substr($tarball['PEAR'], $dot+1);
 
 include_once 'Archive/Tar.php';
-$tar = &new Archive_Tar($tarball['PEAR'], $have_gzip);
+$tar = &new Archive_Tar($tarball['PEAR'], ($have_gzip === FALSE) ? NULL : $have_gzip);
 if (!$tar->extractModify($ptmp, $pkg)) {
     bail("failed!\n");
 }
---------------------------------

You are a star.. this worked for me as well.. the pear is ripe now ;)

This bug has been fixed in CVS.

In case this was a documentation problem, the fix will show up at the
end of next Sunday (CET) on pear.php.net.

In case this was a pear.php.net website problem, the change will show
up on the website in short time.
 
Thank you for the report, and for helping us make PEAR better.

Archive_Tar was expecting null, not false, if compression
was not explicit.

Great catch zae26305, I implemented it more simply by assigning null to $have_gzip, since null evaluates to false and does not raise any notices.