Macworld: "PowerPoint has caught up to Keynote in many areas and surpassed it in some. But Keynote’s workflow and overall feature set remain superior. Practically speaking, your choice of software probably depends more on the hardware you’ll be using to deliver your presentation (and the software installed on it) than on the features of your authoring program. For that reason, unless Apple releases a Keynote player application for Windows, which isn’t likely, PowerPoint may be a more sensible choice for many speakers." - Interesting wrap up. It looks like Apple has done a great job, as usual, of catching up to the leader, Microsoft in turn has responded. The idea of a Keynote player for Windows isn't such a bad idea, do one for Linux as well to cover all your bases.
The bottom line: Keynote 4.5 mice, PowerPoint 4 mice.