Useful links:

By clicking on these links a new window will open. I'm in no way responsible for their content!

www.passionasia.com (a lot of trips can be found either biking, hiking or soft adventure)
www.clickandtravelonline.com (only interested in groups)
www.mountainbike.in.th (nice pictures of a Thai group exploring offroad)
www.cyclingthailand.com
www.thaieco-adventure.org
www.spiceroads.com (offers 14 days trips)
www.northerntrails.com
Bangkok Hash House Bikers They meet once a month for trips...

You may find more, but a lot of websites contain the same itinerary. I found it useful to use Yahoo Asia as search engine.