I installed 10.6 last night and figured I'd post a few notes.
First, regarding licensing: I bought the $29 upgrade, but the label says "Mac OS X 10.6 Retail, there's absolutely no mention anywhere in the included literature that this is an upgrade, and it installed on a blank drive without any issue.
Rosetta is an optional install, which you have to manually add.
Once you install 10.6, you cannot launch Parallels 3. But the Parallels 4 installer won't let you install until you launch v3 and shut down your virtual machines...
GPGMail is incompatible with 10.6, and the developer plans no further updates.
The changes to Expose and Stacks are very nice. Stacks are definitely faster than they used to be.
Changes to the Services menu will make it a lot more useable.
Reds look different. I suspect this is a result of the gamma change to 2.2?
The text substitution works nicely. Looks like it supports up to 11,358 characters. Seems like an odd figure.
Text substitution automatically replaces three periods with an ellipsis. Something you might want to look out for if you're presupposing ASCII. However, this shouldn't be an issue, as the only place I've gotten Text Substitution to work is in TextEdit. Not even in Apple's Mail.
QuickTime Player 10 looks nice.