I have Comcast in the Seattle area.
If you don't care to watch any premium channels; the basics are just fine for you, then you can get a tuner card (or networked device like silicondust HD Homerun) and plug the cable straight into one of these and it can tune the cable channels directly in MC. This is what I do. You can supplement this using netflix (either the windows 8 metro app or the built in JR Media Center interface), hulu, youtube, Amazon VOD, iTunes VOD, etc.
If you DO want to watch premium channels, then yes, you will need to follow Yaobing's advice and control your cable box with an IR blaster. I have never tried this since it seems a little too much like a Rube Goldberg device to me, and I don't watch all that much TV. A cable-card tuner would be a better solution, but I don't know if Comcast provides these, and I don't think MC supports them yet.
A "hybrid" solution to this could be using the Xfinity online TV streaming experience. What I mean by this is that you can open a browser on your tv and login to Xfinity online and view content online. You can view content from premium channels that are part of your channel subscription package. Now this isn't a perfect solution because:
-Xfinity does not provide you all of the content for online streaming. They only provide a subset of available shows and episodes for online streaming
-It is not direction-pad remote friendly. You'll need a remote device that includes a touchpad or other pointing device.
-It probably does not provide live sports game feeds