I'm guessing we will see 10.5 about in Q2 of 2007. Here's why:
1. Apple has said they will be slowing down OSX releases.
2. By waiting and not previewing the next release, Microsoft cannot copy and co-opt the next generation of features. They can only copy 10.4
3. With that said, Apple can see what Longhorn has to offer and make sure 10.5 does not fall short. (Hey, it could happen.)
4. Factoring in #2 and #3, Apple has the chance to embarrass Microsoft with a better release at the time of Longhorn--very possible if the latest rumors about an 07 release date is correct.
5. The timing makes sense. Tiger is still in serious debugging mode, and will be for a good 3-6 months. Apple can then get serious about 10.5. A year of development would make it about June to show a developer preview at WWDC and get the new SDK's in the hands of developers. (Also, at that point, Longhorn should have a feature freeze so they cannot copy.)
So then another 8 months to finish development and release. If Longhorn is on time--which Apple must assume--they will not want to compete for news cycles with Windows. (Of course, Jobs is a bold man, so I may be wrong.)
But in general, I think Apple will want to take the OS to the next level, and put a good 12 - 18 months into it for no other reason than to completely leap frog Longhorn since Longhorn is a pretty solid knock-off of OSX in its current state. (Not as good, obviously.)