If you call Jason, his iPhone, Pre, Ion and pants all ring because he has Google Voice. The service's biggest downside is that you have a new number to deal with—but TechCrunch says number porting is coming.
That means you'll be able to port the phone number you've had for 5 or 10 or 100 years to Google Voice and use whatever phone you want, whenever you want, on whatever carrier you want, with your current number—meaning you'll never have to worry about your phone number again. Right now, you can kinda hack it by forwarding calls from your current number to your Google Voice number, but you're stuck with pitfalls like texts not being forwarded. Google's also got an app cooking that'll route outbound calls through Google Voice as well, so the service is more seamless—currently, your outbound caller ID is whatever's actually assigned to the phone you're using. Update: Lifehacker pointed out a few months ago that Google mentioned the possibility in their support pages, but TechCrunch specifies Google is testing it now and that it'll roll out later this year.
I have to say, it's been interesting watching carriers become more and more irrelevant over the last two years. Their fears of becoming a "dumb pipe" are certainly coming true, and cutting the core of your mobile identity—your phone number—completely out of their hands has to be at least a little bit scary for them.
Now if Google would just solve GV's other major flaw—actually letting people in. [TechCrunch]