Since ISPConfig 3.0.5 now supports custom/additional PHP versions, I took the time to compile some for me today - based on the ISPConfig manual (yep, you should buy a copy!).
Since this might be interesting for others as well, please find attached the resulting .deb packages.
To install, just unzip the wished version(s) and run dpkg -i php-*.deb to install the package. It will then be installed to /opt/php/<version>. Additionally, you should copy the provided .ini files to /opt/php/<version>/lib and copy php.ini-production to php.ini (within this folder).
Note: They were build on Squeeze 64-bit for FastCGI!
Downloads (if they are offline right now, try later -> uploading)
You can add them to ISPConfig now (consider docs/manual to see how to do that).
A very detailed tutorial on how to create your own packages will be published soon as well. Stay tuned!