Does anyone know if there will be an experience cap for NWN. I would like to use the same character through all the modules and single player game but it wouldn't be too much fun if you cap and therefore don't gain any more experience for completing other modules.

From the FAQ:

How many levels can my character gain?

In the initial release of Neverwinter Nights, you will be able to develop your character to 20th level, the highest level supported by 3rd Edition's core rulebooks. Unlike many other RPG systems, a 20th level character in D&D is a force to be reckoned with and reaching such a level is quite the achievement. In future expansions, the level cap may be raised even further.

http://www.sorcerers.net/Games/NWN/nwnfaq.htm#playerchar