There's a simpler way to do it than using a bunch of relays to reverse polarity. What you're looking for is an H-bridge. It lets you use logic pins to reverse the polarity.

I did a similar project to control a motorized ham radio antenna mount on my truck. The mount came with this really silly-looking [0] controller switch that looks like a Hello Kitty product. I didn't want to velcro that crap on the dash of my ex-British military Land Rover so I built this board [1] to control the motor. It uses the SN754410 H-bridge and a Atmel ATTiny2313 that I flashed with Arduino code. I'd be happy to share the source and PCB with you if you want, jwz.

[0] http://www.flickr.com/photos/69697531@N00/6236671758/in/phot...

[1] http://www.flickr.com/photos/69697531@N00/6643770831/in/phot...