It can be done using a Boss SYB5 and an expression pedal to control the speed of your 'wobb'. I actually prefer the Behringer BSY600, as it does some stuff better than the Boss (for my needs anyway).
This video (from about 1'12") is a pretty good demo of what's possible. It's a Drum and Bass clip, but you'll get the idea.
[media]http://www.youtube.com/watch?v=QMbP3Yr2nmk[/media]