Apple hasn't seeded too many builds to developers so far. Hearing from several (3rd party, not Apple) developers, the most recent build (already a couple of weeks old by now) has still had quite a bit of issues, but generally works alright.
From past releases (10.4 and earlier) we know that Apple internally is quite a bit ahead of what ADC Select and Premier members get to test, so we don't really know the _current_ state of Leoaprd. 9A377a is certainly _months_ older than what Apple's working on internally.
The really interesting stuff is that Steve Jobs has promised a couple of secret features for Leopard that still haven't surfaced in developer builds. The question is: What can they be. They can't be core system features, because those would have been put to the test for developers also. So those are probably set. What Steve probably meant was applications and add-ons. Things like iChat. When iChat was new (only an example!), that didn't have any effect on 3rd party developers testing the core OS. Similarly, Apple hasn't talked about iLife '07 and iWork '07 so far. Although it would seem like a stretch to call these "Leopard features", Apple might put iLife into Leopard, i.e. it would only run on Mac OS X 10.5, use its new features (Core Animation etc.) and come free with the OS (instead of free with new machines as has been the case with iLife '06 and earlier).
My _guess_ is that Apple will release Leopard some time _before_ WWDC in June. A May date is not totally out of question, although that would mean we would see more frequent seeding in the next couple of weeks. Final Candidates would have to appear near the end of April at the very latest for a May release.
Any talk of Leopard being late as in "later than June" however, are totally off in my opinion.