It's true, phones such as the iPhone 3GS have built-in GPS and there are good apps for tracking your rides: RunKeeper and MapMyRide.
One issue I've had though is my Garmin eTrex HCX does not have a handlebar mount! I bought one at REI, but the HCX has a memory card slot which causes an indentation in the back cover. Surprise! The handlebar mount replacement cover does NOT have that indentation and is not compatible with my unit.
So what I usually do is just turn on tracking and download the route later. Good for seeing where you've been, not so good for figuring out where you are or where you want to go.
Cheers, Mark