Looking forward to the 10.5.7 update. I doubt that this will be the last release of Leopard prior to Snow Leopard here's why.
I don't think Apple will announce the immediate availability of Snow Leopard at WWDC. My expectation is that Apple will demo an almost complete version of Snow Leopard with Apple announcing that Snow Leopard will be released later in the year.
Apple will probably want to release Snow Leopard after the annual Back To School Promotion as a means to ensure a steady stream of Mac sales in the summer. It would then be logical to follow the promotion with the Snow Leopard release. It all ties back to Apple's fiscal quarters. Apple won't want to have a blow out summer with the school promotion, a Snow Leopard release, and an iPhone refresh. They will want to spread some of this out.
Apple's Fiscal Quarters are as follows:
Q1 - October, November, December
Q2 - January, Febuary, March
Q3 - April, May, June
Q4 - July, August, September
The Back to School promotion usually runs from mid June to mid September (give or take a few days). Thus, it makes sense for Apple to release Snow Leopard in conjunction with a series of Mac refreshes AFTER the conclusion of the school promotion and after the hype of the iPhone refresh release. It would be a great tactic to spur sales just in time for the start of the holiday shopping season!
Let's not forget what happened last year when Apple took on a bit too much when it rolled out the .Mac to MobileMe transition while subsequently releasing the iPhone 3G. Apple won't repeat the same mistake of taking on too many things at one time from a product refresh perspective.
Let's not forget about the Windows 7's release either which is also speculated to be rolled out later this year (it's already in RC stage). Apple and Microsoft will want to ensure the other doesn't get an extended period of coverage, thus I would expect that both OS's will be released within a few weeks of each other.
Just my take