This fellow, however, claims Apple's holding back to keep the 4th quarter sails going:
Apple’s Leopard Strategy: Screw Microsoft, Kill Dell, Save the 4th Quarter Page 1 - Talkbacks - Digital Trends: "I've read hints of most of the above from usually reliable source, and, more importantly, everything but the better security seems entirely likely. The 'vastly better security' promise is intriguing. Will Apple bundle an antiviral solution? They desperately need to do that.
Saving the 4th Quarter
... Another of the primary reasons Apple isn’t being forthcoming about Leopard is the fear that if people get too excited about a product coming early in 2007 they will stop buying in 2006. So, Apple is intentionally not telling you about the great multi-media features in the new product, the security enhancements that will make the existing line obsolete, or the massive jump in application performance on what will be a fully optimized product on the then current Intel hardware.
Certainly you’ve been left in the dark about enhancements that will increase notebook performance and battery life, allow you to seamlessly move between 802.11n, WiMax, and Cellular data networks, and even more quickly create peer-to-peer relationships on the fly.
The UI improvements that better make use of the then current enhancements in graphics technology are hinted at but you won’t see the real power until the OS is released when the true power of the visual experience can be a real surprise (and it is believed to be stunning).
If you get too excited about what is supposed to be an incredibly amazing product you simply won’t buy a new Apple this year. That wouldn’t be a good thing because Apple would like you to buy both years, if possible, and that means keeping you in the dark about what is coming. It is a typical Apple after all...
Even so, it is very unlikely that 10.5.0, or even 10.5.1 will be a good upgrade option. You really don't want to buy a machine with 10.5.0 on it - Apple has a history of major bugs with the initial OS release.
Best to buy 10.4 and wait to 10.5.2 to update; you'll avoid a lot of pain that way.