While this probably wouldn't be of much use to me--especially since I don't have an iPhone, my managing editor (Nick Schultz) pointed this out and I thought it was kind of neat.
Some of the functions might be a little redundant of your Harley's existing gauge functions, but the GaugeFace app/dock ($250) is pretty cool. Simply plug it into your ECU, run the cord up to the dock (mounted on your bars) pop your iPhone or iPod Touch in and you've got a speedo, tach, engine temp, gear indicator, and turn signal indicator all while charging your phone/iPod. Just don't forget to remove it when you get to the bar, or someone will definitely remove it for you.
Now if you really want to trick out your bike with an iPhone or iPod Touch and don't want to clutter it up with a handlebar mounted dock, then check out the DashLink from Hell's Foundry. I talked with Derick at the Cinci V-Twin dealer show last year and he told me he was working on this. Very cool and really clean integration. The DashLink replaces your stock dash and is available in chrome, gloss black or carbon fiber for $399.95. Simply remove your existing dash, plug the new DashLink into your bike, mount the dash to your bike and your done in about 15 minutes. The DashLink charges your phone/iPod, and you can run your navigation app, music app (volume can be controlled through your handlebar mounted switches on FLH/FLT models), and it comes with a cover to protect it from rain. With dashes available for most Harley models, the possibilities are endless.