Mastodon are the "latest thing" in metal apparently - the music press has been wetting itself over them since their second album (Leviathan, which is bloody good by the way) came out last year. They mix trad metal, grindcore and a bit of hardcore. Their stuff is actually really good. They're great musicians (the drummer is f*cking brilliant) and they have some excellent songs. Well worth finding their albums: Remission and Leviathan
Velvet Revolver is basically G'n'R (Slash, Duff McKagan or however you spell it and someone else) with Scott Weiland from STP on vocals. I haven't heard any of their stuff, but they're meant to be pretty tidy.
In Flames are/were part of the "Gothenburg" death metal scene, but they've gone more melodic/proggy in recent years. Great band.
Nightwish are something of a phenomenon. They're a "symphonic" metal band - guitar/bass/drums, lotsa synths and a classically trained singer. They've just got "big" in the last year or so with their most recent album "Once", which has sold massive numbers of copies (it might even have gone platinum in Finland). They've spawned a string of mostly crap imitators, and they tend to polarise opinion - people either love them or hate them. I think they're pretty tidy. Best albums IMO are Century Child, Once and Oceanborn.
Incidentally, doom is the genre that is grossly underrepresented at the festival ('cept for Sabbath, obviously), along with black metal (none at all

)
Kalfireth: I want to go to Reading as well (MAIDEN!!!!!11one

*wets himself*), but finding tickets is proving rather tricky....

Oh, and Suicide Girls - niiice...

Have you seen the Probot video for the "Shake Your Blood" single? That was cool...