In recent months Apple has sped up the delivery of Leopard updates. 10.5.3 was in the end of May and 10.5.4 at the end of June.
At this rate 10.5.6 will be released by the end of the year.
Historically, the pace of Leopard updates closely matches the pace of both Panther and Tiger.
And yes, 10.5.6 should be out by the end of the year. That would be 14 months after Leopard's initial release; for Tiger, 10.4.6 was out in less than a year.
So when they release Snow Leopard will they continue to update Leopard or are they trying to get all the bugs worked out so they don't have to continue support?
Unlikely. The only OS X update that came after a subsequent major release was 10.4.11, released three weeks after 10.5. Apple has never seen an advantage to continuing development on old OS versions.
You'd think they'll add support for Mac to MMe sync!
Also unlikely. I think that would involve some major changes. We'll probably see it sometime, but I would guess that Apple will want their bad press over MM to die down first.