[southernsugar]

That's really interesting to me... At my campus, I would certainly FLIP OUT (and I have... I ripped the letters off someone's car once knowing they weren't in my sorority) if I saw another girl wearing my letters and she wasn't a member.. but as to guys letters... I own a ton of shirts and jackets from events or rush shirts.. Last year one frat made vineyard vines rush fleeces, which happen to fit most girls I know and be VERY comfortable, and they're a popular item to see on campus.
It's common around here for the guys to give girls they know rush shirts- I own phi delt crest shirts, sig ep crest/logo thing shirts, pi kapp shirts, sigma nu shirts, beta shirts, our locals shirts, ato shirts, k.sig shirts, etc.. Lambda Chi came and dropped of a ton of shirts for every girl in our chapter, kind of a PR thing. I know our fiji shirts do say FIJI instead of their letters, but most groups don't seem to care here.. Granted I'm an ex-social chair and do have good friends in most of those groups, but I can think of only one occasion where I've seen a girl asked not to wear a guys letters (and there was a very good reason for it).
At the same time, I've seen a lot of guys wear our letters around- boyfriends, guyfriends, brothers.
I'm not talking event shirts- those are generally a different catagory. if it says "xyz crush" or "event by xyz" then really, I don't care who wears it. We have a lot of GDI girls who wear the 'girls fight back' shirts that say 'sponsered by xyz(written out) and panhellenic.' and that's fine.
And while I certainly wouldn't wear them if I was looking like crap that day, I wear them pretty often. Of course, I wouldn't wear my own letters on a day I decide to roll out of bed. Saying that, unless I was engaged to a fraternity member or their sweetheart, I would NEVER wear their drop. That will get you in pretty big trouble here.

edited to add: Actually, one thing that is really common with those shirts is when an organization comes to another meeting (frat plegdes serenading us, a group thanking us, someone just trying to get our attention to something going on at our campus, etc) they often wear our letters or make letters for us.. We currently have about five sets of GIANT PAINTED wooden letters in one of our storage closets from different fraternities. They're great to put out during recruitment/homecoming/ (dry) tailgates and such.

As for perps... we have one girl who SWEARS she is a member of our sorority, and tells fraternity guys she is. We recently started making friends with a group and they brought her up as an example of why they weren't too interested to mix. The new class said, "who?" and some of us were like...'she's never, EVER been a member of our organization.' but it still makes it hard to see her on campus.