I know this is a forum for BG2, but, is there a program that allows you to design or modify items for P:T? I know that some people (like GigglingGelugon) have made custom items for this game, so presumably an editor exists.

Couldn't rightly say on that score my friend. Try here - http://mud-master.com/

or here - http://www.teambg.net/teambg_com/cgi-bin/YaBB.cgi

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It's bleedin dark down here, slippery too.
http://www.contrabandent.com/pez/cwm/cwm/piss2.gif Man, that's a pisser. http://www.3dpcgames.com/cwm/s/cwm/cwm/piss.gif

Wgy isn't there a P:T forum in this site? There should be.

Yes, an Ironworks forum for P:T would be great! It's still one of the best Infinity Engine games, second only to BG2 I think. The storyline and roleplaying aspect is better than BG2 (IMHO) though it has less replayability, as you can't create your own character(s).
An interesting thing I found today when playing it, is the spell Missile of Patience (which Dak'kon teaches you), it is supposed to be useless according to the description, but is actually very powerful - it sometimes just fires a missile doing 12 damage points, but about half the time it summons a magical bow (much like a bow version of Mordenkainen's Sword) which fires ice arrows and lasts for about 20-30 seconds.