On cell phones, Caller ID shows the phone number on an incoming call. If a person is in your address book, you get their name. Otherwise, it's just a number.
Contrast this to landlines, which have included name information on caller ID for a long time now.
MetroPCS is bringing landline-style caller ID with name to your cell phone. Pretty darned cool. The price is reminiscent of landline providers' offerings as well: $2 extra on $35-$45 plans, free on the top-end $50 plan.
So if you miss having a person's name show up on caller ID, you can now get the feature on an unlimited cellular plan. Very cool, though the extra charge for the feature reminds me of AT&T, Verizon etc., not a good thing.