Woah :) I don't have any official word, just that it has been stated previosuly that our goal is the current version plus one previous, and that postgresql 7.4 will be out shortly.
I don't think there are any major issues to supporting 7.2 in openacs 5.0 though. What is the offical opinion?
I will write up the exciting journey from OpenACS 4.5 - 4.6.3 while upgrading from Postgresql 7.2-7.3 for OpenACS.org soon.