Apple has updated a couple of knowledgebase articles, one of which covers how to tackle problems with Mail connectivity in OS 10.6, and others that discuss the enabling of Magic Mice gestures and how to prevent a specific iMovie crash in OS 10.6.3.
This article is rather detailed and covers a number of settings and methods for figuring out where connectivity problems lie in your email setup. It starts with basic setup requirements and troubleshooting with Mail's "Connection Doctor," but then covers managing previous recipients to ensure addresses are correct, and testing your provider's servers for connection problems.
Some people may have trouble getting their Magic Mice to work on their systems. While the mice should pair and function in OS 10.5.8 (with the Wireless Mouse Software Update 1.0) and Snow Leopard, Apple's recommendation in cases where problems occur is to upgrade to the latest version of Snow Leopard, and then pair the mouse again.
Apparently when running iTunes in OS 10.6.3, a number of people have been experiencing crashes when using the Titles window to apply text overlays to their movies. According to this article the problem lies with the Far Far Away (star wars) themed text, and should be avoidable by either downgrading to OS 10.6.2 or avoiding displaying the theme in the Titles browser window.