|Mike's method will indeed work, but there a couple of subtleties to keep in mind.|
1 - I believe that using =0 will cause Excel to recalculate the cells just like all the other formulae. In most cases this probably won't matter, but in a large spreadsheet with a large number of formulae, it could slow it down.
2 - When using the Tools...Options...Formula checkbox, these cells will show up as formulae. Probably not a big deal, but worth mentioning.
A method that avoids these issues is:
1 - Put a 0 in one of your blank cells
2 - Copy the cell (Ctrl-c or right-click...Copy)
3 - Select your range
4 - Use Edit...Paste Special...Operation: Add
Of course, both of these methods require that you have the Tools..Options...Zero Values option selected.