While PHP 5.2.8 is the hard (required) min version to run Moodle 2.0, I think it could be interesting to add one new environmental test to be executed on each install / upgrade:
- Create it as custom and optional check.
- Make it look for PHP 5.3.3
- If the test pass, remain hidden.
- If the test fails, inform about the benefits (memory/speed/security) of using PHP 5.3
- Make the test available also in 19_STABLE (but apply it only to 2.0 version)
I think it can be interesting to have that being informed continuously to admins. Differences are huge enough.