You may be searching such things that are not reported in these forums - http://docs.moodle.org/dev/Developer_documentation is a good place to start getting familiar with development of moodle ...
Developers of moodle as well as the whole community here is mainly focused in testing code of moodle itself and the latest releases of moodle in various environments, not so much the latest versions of databases or the latest versions of php as such. The same way if you find some tinymce bug you usually first report it to moxiecode (tinymce bug tracker) and try to get it fixed in core tinymce - and if that is not possible try to get a fix / work around to core moodle code.
I suppose the answer that Tim gave just meant that he thought there might not be any obvious reason why postgresql 9.1 would not work as well as postgresql 9.0 does with moodle 2.1...