Here's a list from Barnako's site: http://barnako.typepad.com/news_of_st_j ... index.html
Lots of cyber cafes with computers to use, maybe less common to find wifi, other than the Cruz Bay spots. I found Keep Me Posted very good for grabbing a drink at Aqua Bistro, and sitting right next door and connecting on their link, or you can use their computers for just a couple of bucks.
Donkey Diner looks for a small donation and it's easy to work on your laptop while waiting for breakfast. The tables are large and the staff is very friendly. They also have hard wired computers that you can use if you don't have your laptop.