DCT 700

Doesn't it have to be provisioned by the Cable Company to be able to decrypt the stream?

Yes

They do make decent bookends, but I doubt any US cable co would activate a purchased one. I hear they might in Canada. Call your cable provider and ask.

I believe CableCard is free with Comcast, but you will have to build your own DVR (if you want multi-room experience) or get TiVO and pay $20/month, which is what you are trying to avoid in the first place.

With a CableCard you can potentially view 6 different programs on 6 different TV's if you go with Windows Vista/7/8 solution.

Comcast FAQ about Cablecard fees:
http://customer.comcast.com/help-and-support/cable-tv/about-cablecards/#cc9

Look at the "note". If I'm interpreting correctly, it seems to me that they may/will charge a monthly additional outlet/drop fee.

Only if you need to connect additional TV's using their service. With a single CableCard in a Windows system you can decrypt 6 streams (with approproate tuner) and then distribute them your self over the home network, without having to pay for additional boxes, or outlet fees.

You literally only need one cable drop, to connect to the tuner, and the rest can be done over network.

I believe that's what it becomes once the DTA is replaced with customer provided equipment and a cablecard. It's best to contact Comcast to get clarification of their pricing policy.
The OP is asking about cablecard to replace a DTA on one TV. If he/she is interested in building an HTPC and networking that is always an option (and probably more expensive than a DTA or cablecard). But..... if interested, that should perhaps be another thread.