Based on my experience so far with the iPhone 3G, and several seasons with other smartphones such as Blackberries and Treos, I would say the answer is NO, the iPhone cannot ski that well.
Other than biffing on it, the thing that will kill a phone when skiing is moisture.
I recommend either using a phone you don't care about, or putting your phone in a ziploc.
High tech breathable ski clothing wicks moisture away from your body so that you don't get cold... and it condenses where your phone is, on the inside or outside of your outer shell.
For light use, in fine weather conditions, an iPhone will work. But if you are agressive, get hot, or ski in extreme conditions, I recommend taking precautions.
The iPhone 3G has a additional weakness: it does not do well with moisture when operating with the headset.
http://discussions.apple.com/thread.jspa?threadID=1768715&tstart=15
I have a lot of experience faking work while skiing. emailing from Blackberries and Treos on chairlifts, listening to conference calls on mute while skiing pow on the way down...
...and I ski with a junker phone, because I don't care if it breaks.
My iPhone stays in the lodge.