Couldn't you already just plug a fairly cheap USB dongle in your rasperry pi ? I'm not sure I see what the big deal is, aside from probably making the price cheaper and freeing up a usb slot
If you've ever tried to get wifi working (esp on earlier RPis) you'll understand it is not that simple. Especially for newbies. Having said that its a good baptism of fire.
Raspberry Pi with built in wireless comms means it uses a standard piece of hardware so the software stack can be configured to work out of the box. That can't happen with the existing arrangement. Also the power supply problems with original Pis are frustrating to say the least. These are most evident when plugging in USB wifi devices. Again having a standard integrated piece of hardware will (hopefully) avoid this.
You're fairly limited in terms of antenna size/placement when you use a tiny dongle. If it's built into the Pi, they could have the antennae wrap around the edge (similar to how the iPhone does it) to allow for a much higher quality connection.
At least, that's what I hope they're doing with it.
Raspberry Pi with built in wireless comms means it uses a standard piece of hardware so the software stack can be configured to work out of the box. That can't happen with the existing arrangement. Also the power supply problems with original Pis are frustrating to say the least. These are most evident when plugging in USB wifi devices. Again having a standard integrated piece of hardware will (hopefully) avoid this.