I know exactly what you're trying to do, and I also play GW and have done it. The previous poster is correct. Using Crossover Games will allow you to run Guild Wars within OSX. You can even copy your old .DAT file into the directory that Crossover makes for Guild Wars, and paste it there so that you don't have to download the entire game all over. You will have to search for that specific directory though, since it's pretty deep into HD/Libraries/etc. There was a period of time earlier this winter that Crossover had a promotion where you could download the full version free, and I'm sure there are copies of it floating around the internet in places. Also, an additional benefit of Crossover Games is that you can run alot of Windows programs that aren't officially supported by CG, since the compatibility files provided by the program support execution of Windows programs that aren't too heavily system file dependent.
However, there are a few things you should keep in mind:
1) You'll have significantly lower framerates in GW within OSX. Typically around 15-30fps. You'll also have to turn down some of the settings.
2) Sometimes you won't end up in fullscreen. Be careful when minimizing/maximizing windows, since that can sometimes cause graphics glitches.
3) You'll also have to change the function keys within OSX system preferences if you use F1-F4 for weapon set switching, since the keys are still mapped to your OSX hotkeys for expose, dashboard, and etc.
While it may seem like a good idea, I'd strongly suggest you bootcamp GW. Doing that will let you run it at max settings, with almost a constant 60+fps. The only time I ever do GW within OSX is if I just want to chat with some people or don't have the time to swap into XP through bootcamp.