Unless things have changed recently, if they accept your submission you do give up some rights as to being able to show your movie elsewhere. This could be as simple as saying that you cant put it up on AtomFilms or something similar, I don't know....just make sure you read the rules and understand before going through with it.
I got my Zombie Fest movie on ifilm. From reading the rights thing. They can use it as they like but you can still show it where ever you want. They just dont want liabilities if you do any copyright infringments and I think alot of the garble is about that. But it clearly says you can still use your video anywhere else. Current TV wants all rights to you joint so watch out for them. There are a few others also.