There's a cheap electronics package that works well and is bundled into different casings and rebranded multiple ways. Here's two at Nashbar:
http://www.nashbar.com/profile.cfm?c...Cyclocomputers
http://www.nashbar.com/profile.cfm?c...Cyclocomputers

It works fine except that the historical max speed feature is prone to glitch and claim that at some point in the past you went 88mph.