I made a standing desk out of a cheap shelf system back in January [1], and haven't looked back. About the only change I've made since putting up those pictures is swapping out the Apple keyboard for a Kinesis Freestyle.

I'd like to build some small shelves off the sides of the top level to add some speakers, but beyond that I've been extremely impressed with making the switch.

[1] http://elasticdog.com/2011/01/shelf-made-standing-desk/