Misty, my rescue Pointer, loves mountain biking with me. She's a little skittish of everything at first so it took a few tries and some doggy treats to get her warmed up. The other day I opened the back of my truck to get my bike out and she jumped in, all psyched for a bike ride. I couldn't say no of course and brought her out again.
Before Misty I had a very wild and fun loving Dalmatian Mollie. She would have loved this springer for dogs that I found on Kamala Dolphin-Kingsley's photostream. I love this video. Also, check out her flickr stream here for other fun dog and biking pics.
P.S. Misty mountain bikes without a leash. This looks like a fun option for some dogs to go biking though.