What has happened in the past is that people volunteer to test things out for new versions and just let the community know the result. Frequently the same people volunteer to fix any problems. We have a similar situation with Oracle 9i and there are a couple of people who've volunteered to fix OpenACS so it works with 9i for the 5.0 version. For PG, I and a couple of other people volunteered to make the necessary changes for 4.6.2. Some things were missed, unfortunately, but I think 4.6.3 works quite well with PG 7.3.3.
However ... the toolkit's big and we don't have regression or coverage tests (both things I'd like to have happen in the next few months) so testing's a big job.
I've been using PG 7.3.2 for all my OpenACS development work for a few months now so I can say that the core and basic .LRN functionality work with 7.3.x just fine.
