Visited the US recently for the first time too, and was a bit confused by all this. I ended up doing the following - whether it's 'cheap' of me or not, this is what I felt was reasonable from what I'd read:

Restaurants: 20% tip was a starting point, but I marked it down for flaws in the service more easily than I would in the UK.

Hotels: Always took my own bags (wasn't staying anywhere fancy), so no worries there. Room service gratuity was as wbl-mn-flyer says above, included in charge - but I sometimes gave the guy a couple of dollars anyway. I always left two dollars on the bed for the maids before heading out for the day. Don't know if it 'maid' (aha) any difference to the service I received, but sometimes my towels were folded nicely(!).

Taxis: Always rounded up to the nearest dollar, minimum one dollar on low fares, sometimes an extra dollar thrown in.

I disliked having to tip the hotel staff that handled the buffet breakfast, but that's just me. I don't think they do enough to earn the tip, but at the same time I'd personally feel too cheap just leaving one dollar so I always left two.

(OT: I got handed a lot of empty taxi receipts, which I'm never sure how to handle for expenses. Am I supposed to try to mimmick a real taxi driver or just use my own handwriting?! I ended up doing a bit of both...)

[sarcasm]PS: In LA, you get the added bonus of a feel good factor for supporting all them out-of-work actors and actresses with your tips[/sarcasm]