The US has a 110Volt 60HZ supply (the UK is 245/250Volts at 50HZ) and uses a two flat pin plug. I worked in New York for 6 months as an engineer for BT and have holidayed there and Canada for many weeks. What do you need an adaptor for? If it's just chargers for a mobile phone, camera and a lap top check the printing on your charger. All mine quote an input of AC 100-240 V 50-60 Hz. so the voltage and frequency is within the devices range. So you only need an adaptor for the physical aspect of plugging into the socket!
If your talking of hair dryers, irons etc. then yes you should have a voltage adaptor/transformer. Personally I wouldn't bother. Check with your hotel web sites, most provide these items in your room or at least have some for guests to borrow.
Another point worth checking before you go, is your mobile telephone dual band, triple band or multi band? The US mobile networks use an operating system different to the UK and the EU. Dual band phones don't work there. I use a triple-band phone and have never had a problem. You cannot adapt a dual-band mobile for triple-band systems. If you have still got your operating instructions check the specification, if not check the company's web-site. Any other queries just write back and I'll try and answer.
PS Using a 240V appliance in a 110V socket is not intrinsically unsafe, the lower voltage will cause hair dryers to be cooler etc. However using a 110V appliance in a 240V supply is extremely dangerous. Also, if you buy video tapes and DVD's check that they will work in the UK as again the US uses a completely different system.
Have a good time.
Peter