For progression raids and the like, RP is usually non-existant. Albeit exceptions do occur. For older raid content however, you can make a ton of awesome events with the guild, and as such this form for roleplay in raids is way more common. - Also because people that take interest in this kind of RP rarely sit in the same progression guild. Frankly, its hard to find players good enough to progress and roleplay.
Depends on what you consider 'RP'. Sometimes when waiting for the group to catch up or during breaks or such, me and a couple of my guildies act as though we're in character, but with every feature of the chars' personalities driven to the extreme. Then the dirty jokes start and there's no going back anymore.
Then again, we also do that in public channels from time to time. x)
I don't RP in the same group I raid with. Though DS normal hardly is progression. I generally don't rp in vent anyways, I usually limit myself to the means in game to RP. (yes wow does have a voice chat, but no.) In a raid I usually just chat during trash or in-between boss fights, or talk about random nonsense in vent. Occasional RP might happen in /s /w or /y however even during a raid.