Depends on where you live. Comcast uses different DVRs in different markets.
If your Comcast uses Scientific Atlanta equipment, then they offer the SA8000HD that will let you watch one channel while recording another; it will also let you record two different channels simultaneously.
If you live in an area that uses Motorola equipment, they currently offer the Motorola 6208 that can only tune or record one channel at a time. However, between now and February, Comcast will deploy a new Motorola DVR (model 6412) that will let you record one channel while watching another; it will also let you record two different channels simultaneously. When you call to inquire about their DVR, tell them you want the 6412.