Keep in mind the transition to DTV is for the USA only, so only American broadcast signals will be affected. Canada makes the switch in 2011, I believe.
There should be no change in aspect ratio; shows currently shown in 4:3 will most likely remain the same. Same goes for 16:9 shows - some of these channels/shows might be standard def, others might be HD...you'd notice a difference for sure.
I think there are a couple of previous threads on the switch to DTV if you do a search.
Panasonic TH-42PE7U plasma, Toshiba HD-D3, PS3, Shaw DCX3400, Yamaha HTR-6160, Polk 6.1, Belkin PF30, Harmony 700