In reply to CharlieMack:
I'm off in Sept/Oct this year, but I'll probably just spend October in Yosemite, off to Tahoe first.
Flights cost me £680 to San Francisco from Manchester (via Philadelphia), hire car for 7 weeks cost me £730. BMC insurance was £230.
If you just plan on being in the valley, you can save a lot of money by getting the bus there (via Merced I think). You can use the valley buses to get around a lot of the crags, but it won't run all the way down to El Cap, so you might have to hitch for some areas.
Camp 4 is $5 a night (or at least it was in 2011).
EDIT: You can apply for an ESTA online (US visa waiver) up to 2 years in advance and the cost is minimal (can't remember exactly). It will give you 3 months in the US, but you'll have to book your flights first (they need your flight details). Likely you will be accepted immediately (unless you declare yourself to be a terrorist).
EDIT: There are no showers at Camp 4. You can get a shower at Housekeeping (get the bus towards Curry Village), take your own towel and sneak in (or pay $5 and borrow a towel), or get the code for the showers in Curry Village (they're for the campers at Curry Village but it got passed around by word of mouth, not sure if this is usual). Or don't shower.
EDIT: The Camp 4 toilets can get a bit messy. If you fancy a bit of morning luxury, wander down the road towards the village and use the tourist toilet, a lot nicer. Or even better, go over the road and sneak into the Yosemite Lodge toilets.
Post edited at 10:58