The wife and I did it in September of this year. Organised by HC travel who advertise in many mags. Picked up Harleys in Chicago and rode to Santa Monica pier. Had a wonderful time. Hotels all organised in advance, paid for with vouchers which we received together with our air tickets and itenerary suggestions a few weeks before setting off. At the risk of boring everyone there are a few things to consider.
Time of year. Late August into September is said to be the best time. Before this it can be too hot. Later and the weather particularly in Arizona can be cold. Too early in the year and it can be unsettled.
Arrange it yourself? Easily done, especially if you've been to the States before. Eagle Rider will provide the bikes, and you can pick your hotels/motels on the road. From next year Eagle rider will be providing Gs1200's for hire at the same rate as the Harleys ( $130/day ). Personally I would opt for the Harley. It's a question of horses for courses, they ride beautifully on the long straight roads, and the fellowship of other Harley riders has to be experienced to be believed. Even with the benefit of hindsight, if we were first timers we would have an organised trip. It helps you to get the most out of the trip, and miss as little as possible.
If you haven't ridden a Harley before it's worth a trip to a local dealership to sit on a few, and decide the best model for you. You can get a feel for the models with feet forward controls etc. We decided on a pair of low riders. They don't have feet forward contols ( although they do have feet forward posing footrests ) and we both felt that if we needed to stop quickly we would prefer to have the footbrake in a familiar position. Only downside is that there is no screen. No problem up to 65-70 mph, but really difficult above this speed. One recomendation is to avoid Sportsters if possible as apparently they can be even more vibey. Having said that the 2005 1200's have a new smoother engine, and also 2 disc brakes on the front wheel!
Luggage. The bikes have 2 smallish leather saddlebags, and we both had Heine Gericke £10 roll bags which we bungied on to passenger seat, resting at either end on the saddlebag.
I would reccomend taking your own helmet. The supplied hire ones are as tatty as hell. You can buy one out there, not too expensive, but not sure of quality of testing or whether road legal in this country.
As for riding kit, if I went again I would take a set of dragging jean type denim thingies. Enough protection, and not too hot. Take waterproofs.
As for money, plastic rules. They take it almost everywhere. Unlike a previous poster I had a few problems with traveller's cheques. They really only like American Express cheques. On one occasion in LA I was refused in 2 major banks when trying to change travellers cheques. In the end used a Well's Fargo change office. Almost felt like a second class citizen.
So many must see's amd must do's so if you have the time stretch the holiday a bit, and take time out to stay a couple of nights at a few places. In particular it's worth a detour to take in the Grand Canyon if you've not been there before.
Loads more if you want it, just let me have your email address.
We had such a good time we hope to go again in a couple of years.
Harleys? 
