You probably already know this, but you CAN run two HDTVs off the 722 TV1 outputs (one on component and one on HDMI). You just have to watch the same show on both. You can control TV1 remotely with a UHF remote. Long cables aren't that expensive if you get them from someplace like Monoprice.com.
Bradtothebone is right. Unless you have the need to watch three different HD programs at the same time, then you are better off running a long HDMI cable from one (or both) of your existing 722s. You'd have access the the 722's DVR content and not have spend a monthly fee ($10 more) for the extra box. You could also buy an extra remote too to leave in the room with the new TV.
That is a great idea but the way my house is laid out Its not really feasible for me to run HDMI or composite cables from either one of my other HD boxes. I'm thinking the extra 10 bucks a month might be worth keeping the wife happy.