We launched 5/8 in '09 and it was 105* at Phantom that day. The whole trip was hot. Only rained on one evening on day 15, but as other have mentioned, bring a extra layer or two.
Double ditto Martin-Whitis guide book for sure. Someone on MB suggested to bring two because they lost theirs in rapid or something. We put ours in one of those clears plastic ziploc map pockets that has d-rings on it. We then put a 'biner and a small loop strap that makes it easy to read w/o un-clipping it. We get the map pockets at REI. Liddemann's is a good guide too. Belknap not so much IMO.
Get a cheap military duffle bag at a surplus store to put your spare beer in. The Chinese/Korean ones are made of a heavy nylon material. Prolly $10.
Get a plastic deadblow hammer (Home Depot) for smashing cans. Way more comfortable than using a rock. Get a feed bag ($2) from a farm supply store to put you smashed cans in so they don't puncture something. BTW we rinse the beer cans before smashing. Our friend brings an old NRS Bill's Bag to put the feed sack w/ the cans in and by the end of the trip it gets a little rank if you don't rinse.
I've rowed our 18' cat and a 18' Sotar raft and they're both real pigs to row in the wind. on our first trip, got a whole 3 miles one day before calling it quits.
Have a blast. We launch on 9/18.