I've found that setting your keyframe rate and bitrate to "automatic" is very good. It's actually the best for Vimeo and Youtube.
UPDATE 7/8/2009: I would suggest the following changes:
1. Key Frame every 30 frames (drastically reduces file size without any noticeable picture quality shift for Vimeo).
2. Set the bitrate to restrict to 5000 kbps
Otherwise, everything else remains the same.
This is for Compressor 3 and starts with H.264 as your compression setting. Depending on how long your video is, you may want to restrict the bit-rate in the video settings tab. That's it!