It is not absolutely true that all those carriers use the same towers-- the fellow was lying or misinformed. Now in some areas, several carriers may share some towers (meaning that big pole that sticks out of the ground) but each of them has to put THEIR OWN panels on that tower in order to broadcast a signal and provide service. The number of panels, type of panels and directional orientation are what determine service quality. There are of course other issues, like the higher the MHz of the signal, the poorer building penetration will be, so Verizon and Cingular have an edge over Sprint and T-Mobile.
I have nothing against Sprint, and their service is generally fine in urban areas. They are also stronger in some markets than others. So it's hard for me to say how good their service will be wherever you are. They generally do not have the strong blanketed coverage that Cingular and Verizon have. But they're better than T-Mo . Yes, Sprint's data plan is a steal, and you get the same EVDO speeds as with Verizon.
I don't know when the PPC-6800 will come out. Hopefully by the fall of this year, is my guess. I also don't know the price, but would guess it will be somewhere around $299 or $349 with a 2 year contract.