Caught up on Chapters 21-25 of Star Wars - Clone Wars on Cartoon Network off of my DVR. Chapters 1-20 are on DVD now, and together the 25 chapters fill in the story between Episode II and the upcoming Episode III. If, instead of just reading the famous text crawl this summer before Episode III, you want to see what actually happens, check out The Clone Wars. The animation and music and voices are top notch, and some major plot points occur between the two episodes.
By this point, the entire story of Episode III is available in all sorts of formats (for example, here), the major spoilers out there if you want them. I've tied to avoid them, but The Clone Wars won't spoil Episode III, they just provide background. In particular, I liked General Grievous, a predecessor to Darth Vader, though Grievous looks more machine than machine, a multi-armed warrior trained by Dooku to eradicate the Jedi. Grievous looks a bit goofy below (are robots bashful? why do they need to wear capes?), but animated in The Clone Wars he's one bad mofo.
The Clone Wars timeline provides key plot developments leading up to Episode III.