PT will never go to F-1 for many reasons.
First he already has his chance somewhat back in 1994 I believe. He tested and had a rather crude contract presented but denied it.
Secondly he is rather old. Granted there have been succesful people his age, but with so many good young talent coming up (Massa, Piquet Jr., Rosberg and how many more) few would be willing to make such an 'old' gamble. Hard habits are tricky to fix...Paul's been doing his for years, it would be harder to make him adapt and change his style if need be versus a 22 eyar old.
Third he really hasnt proved anything. This years talent is low compared to the last few years. No Brack, Neves, da Matta, Andretti etc. etc.
Forth, Im not sure F-1 wants another JV as far as attitude goes
Fifth who would take him or where would he go? He doesnt have the coin to buy a ride with Minardi (even if he did why would he), Toyota has its '04 line-up, Sauber wouldnt be interested as they already have a few young guns coming up (ala Kimi and Massa), BAR is full with talent in the wings (Sato, Manning), Jordan needs either a pay driver or someone to attract sponsors, PT wont do that over there, and all the teams Paul would consider, Renualt, Ferrari, McLaren, Williams etc. already have what they are happy with, wouldnt put up with Paul's personality and simply wouldnt think twice on signing him.
Paul stays with CART until he retires or decides to race IRL or NASCAR.