I've been to Coachella 3 times and camped out at it twice. It's a fantastic festival; probably the best I've been to. I can only do one festival a year for a while so I'm passing on it this year. Here are my thoughts on the festival itself.
Free water availability in the festival is poor to lacking. For sale water is $2 a go. Gatorade was $3, maybe more now. You may not be able to sneak in your own Nalgene but I understand the fest is now selling their own 'souvenir' containers that allow unlimited refills. I drank around 3 cases of water in '07 over the w/end (Thurs afternoon - Mon morning) Food availability in the festival is wide, varied, not outrageously expensive and generally v.good quality.
I ate at the Augustine casino each day before heading in to the fest.
http://www.augustinecasino.com/ Good cheap eats and plenty of it. It also put me on the 'right side' (South side) of the festival which helped me avoid a little festival traffic. It's a little out of the way though especially if you're trying to corral a bunch of drunks on a bachelor part w/end.
Leave the sak at the motel. It will either be taken at the security check or left unused for the w/end. There is a tremendous amount of PD / EMS in the area if something happens.
I wear SPF50 sunscreen. I've seen security take aerosol type sunscreens from people at security check. YMMV. Widebrim hats are a great idea, just don't bring a novelty store sombrero.
Bandaids etc are a good item to bring in. I usually carry a small flashlight too in my backpack as well as some baby wipes and toilet seat covers. Bring earplugs too especially if you go near the Sahara tent.
I had zero issues getting/sending txts within the festival grounds. I believe they bring in mobile towers for the weekend.
I'll edit to add more as I think of it.
Have fun, take lots of pics.
Don't take anything that you can't afford to have stolen.
From my experience there and from reading the Coachella forums for a long time, theft within the festival or the campgrounds is not a major problem. At all. Does it happen ? Yes but a lot less than you would think considering the opportunities people have.
.......... Folding chairs, they are a pain to lug, but you can get them cheap there.....
No chairs.
Highs may be in the 80s, but nights are dropping into the low 50s, plus a breeze, so take something warmish to wear.
Try closer to 100* during the day. I never wore anything in the evening but a lot of folks did. YMMV. Dates this year are earlier than previous fests by ~2wks so I'm not sure how temps will be affected.