I'm typing this from a standing desk: http://dl.dropbox.com/u/1038179/Fredrik2.jpg

It's a "Fredrik" from Ikea, which courteously lets you choose the height of the desk and shelves, and doesn't stop you from putting them in the wrong spots. I've been using it for almost a year, and it's perfectly sturdy.

One caveat: make sure you make it high enough. Using a standing desk when the keyboard is even a few inches too low can be miserable on your back. The desk should be at the height of your elbows, and no lower.