Can you please give us a script that reproduce your problem so we can really check the issue?

Thank you for your report.

I've tried to write a script to regenerate the behavior. I think I've missed something or php.ini settings were cached somehow. Now, everything seems working fine. Sorry for bothering you guys. But I still have a question. In old versions (4.1.x) when giving an absolute path like :
include ("\includesdir\includedfile.php");
everything was working fine. With 4.3.0 the leading backslash is causing problems and makes php report "failed to create stream". But 
include ("includesdir\includedfile.php");
works. 
Is there something I'm missing? If the reason is a structural change, I thought, there should be a switch to set this behavior but couldn't find anything related.

Thanks for the quick reply and I'm really sorry to waste your time.  I'd really appreciate it if you can answer my question.

No problem.

About your question, we now have a new stream handler (thanks Wez). Since you're working on Windows you need to give the complete directory if you are starting with '\' so like C:\includedir\lala\lala.php but I'm sure you should prefer to use C:/ and not C:\ so you will not need to use \\ Sometimes.

If you need more help about the way directories work,please ask in php-general@lists.php.net. 

Thank for your report.