I know it's somewhat poor form to show up on a forum with 0 posts and try to hock you goods. Sorry. Anway... I hope this doesn't infringe/violate any anti-commercial posting rules (if so admins, please delete this - and accept my apologies)
I created an iPhone app called Adaptunes that launched today. It controls the volume of any music you're playing based on how fast you are traveling (via gps). I came up with it as a solution for listening to music while snowboarding but it also works really well for mountain biking -- It basically works the same way some car stereos do in the way that the volume will raise as you go faster.
I've got it on sale right now for .99 (it'll eventually go up to $2.99). The current version is kinda feature-slim compared to the update that should be out in the next week or two, but if you were to get it now, the update will be free. It only works on iPhone 3g or 3gs models and requires the new 3.0 os to work properly. We're working on some really cool features that should really make this a sweet app... adaptive playlists that can pull chill music while your riding slow/uphill and switch to something more rocking as you decend... shake controls that would increase volume based on a designated amount of agitation.... and some other good stuff
If your interested in checking it out, here's web site with more info and a direct link to the app store download http://www.adaptunes.com/
Hope this comercial'ish post doesn't piss anyone off - I thought it was sorta relevant to this forum.