After more wasted time today than I care to divulge, here's what fixed it for us: creating new user groups with DIFFERENT names than the ones from our Panther install. For whatever reason, it seemed that the old BSD permission group names from Panther were somehow confusing Tiger.
I recreated the groups when I first noticed the problem (even after choosing 'convert legacy groups'), but it wasn't until I recreated them with entirely different names later in the day that the problem was resolved.
The odd thing is I wasn't even keen on using ACLs at first since Retrospect doesn't honor that particular metadata yet, so when I first started this ordeal I turned them completely off. But, as it turns out, whether I was using ACL or BSD permissions, the outcome was the same - permissions simply would not automatically propagate from the parent directory to new files and folders correctly until I used a renamed group. Incidentally, I haven't tried it yet with the old BSD permissions - it's working now and I'm afraid to touch it!
Here's the kicker... unfortunately, it wouldn't resolve for me until I manually propagated the ACE of the newly-renamed group down from the root share. This was a fairly painless process for us, as our permission scheme is pretty straightforward and simple, but I can see where this might be problematic if you have more complicated scenarios.
If y'all are having the same problem I am, this may work out for you if your permission setup is pretty simple, other than that you may want to wait on a more definitive statement from Apple. All I can say is that this worked for me.
Good luck
Danny