Pressure doesn't help, but I hardly think Romo is under pressure than most other QBs.
Look how much pressure Roethlisberger gets on him, more than essentially any quarterback in the league. Yet he delivers, using last week as an example, 7 passes to Ward for 109 and a TD, 5 to Holmes, including a TD, 8 to his TE and a smattering to three other players. He isn't keying on a single guy.
Other teams that get beat on at the line, like New England, have had similar success. Cassel gets a guy on him before he takes his third step, but Gaffney, Moss and Welker are all weapons, all people he looks to.
I'm not saying either of these guys is as talented as Romo, but the idea that pass distribution tendencies will always be forced by pressure is absurd. You identify the blitzer(s) and determine where in the scheme the hole(s) may potentially be as a result. Instead of doing this in 2008, Romo has tended to assume it will be Owens or Witten, and when that fails, simply dumps it off to the back for -1 to 3 yards.
Doesn't mean he's a bad quarterback, just means there's a part of his game that just so happens to be bad.