I think it depends on the kind of party. If it's a small get-together, a sit-down sort of event, or a movie night, I would say it's definitely rude, the same way whipping out your phone and talking on it would be (for anything other than a functional message). But if it's more of a house party, with lots of guests and people moving from room to room, a casual atmosphere...not such a big deal. At least, not amongst my social set (nerdy 20-somethings) anyway!
Texting someone in the same room seems like it's a little odd, like they might be talking about someone else, or something. I mean..my friends and I will do that to be funny, when a few of us are hanging out casually, but it'll stop at one message and everyone generally will know what it says.