Yes it is rude. Even with no sound there's still that bloody annoying light that ruins the experience.
I remember once, me and a couple of friends were going to see 'Gran Torino' (y'know, with Clint Eastwood?), and during the climax of the movie one of my friends started to text someone. Proper threw me off the awesome part of the movie, and I ended up being pissed at him for a week or so... >_>
I will however say this though, texting is really rude, but not as rude as talking with a normal voice, about something completely different and ignoring the movie all together. That happened once while I was at the movies with my date watching 'Unstoppable'. It was f**ckin' annoying, thank christ they only spent 5 minutes in the cinema before being tossed out...