Great work! I would be glad to help with anything you need. I was in a similar state a few years ago. Although I didn't use arduino, I do use an Atmel. For me the electronics was the easy part. The hard part is getting the form/mechanical stuff right. Cause no matter how great it works, it won't stay long on the bike if it doesn't look good.
Check out my speedo and tachometer.
www.speedwayinstruments.com
Those are SO sweet. Are the cases all custom made? I hope you're selling a ton of them!