I'm considering shooting my next short film using the widescreen option on the Canon XL2. I've been told that the only way for it to show up correctly on a regular television is to do something in an editor. Is that true? And if so, how would I do it in Final Cut Pro? And would there be any problems if I was also shooting in 24fps?

In FCP, just capture as anamorphic into a 4:3 timeline and you'll be fine.