A great article about letterspacing type while designing for the web. In short, follow a few simple rules...

• Capital letters - it's always good idea to increase letterspacing when using capital letters.
• Font size - as size increases, letterspacing decreases, and as size decreases, letterspacing increases.
• Font weight - as weight increases, letterspacing decreases, and as weight decreases, letterspacing increases.
• Light-on-dark text - light type on a dark background usually looks better with an increased letter spacing