After some research, I bought the Garmin eMap from The GPS Store about a year ago, and have been quite happy with it. It is extremely portable and has downloadable maps (from CD which you purchase). If you go this route, I recommend the 32Meg memory card, as it can store about 4 US regional maps at once.

I've also used it to travel in number of Asian countries, and find it quite a versatile unit overall. I've seen the base unit sold in many of the NYC camera shops.