comcast like fios only broadcasts at the most 1080i. What box do you have. try turning off the box leave the tv on and within two seconds press menu, that will bring up the config menu where you can change the settings.
This is good information to have. I was freaking out because I couldn't get my cable to come in any better than 1080i. From what I understand, this is a matter of bandwidth. Once cable providers come up with more bandwidth, they'll start broadcasting in 1080P.