I'm pretty sure the new iphone 3g has true GPS. I have one and it works quite well. It may use cell towers to get a general location so the GPS doesn't have to "work so hard", use extra battery power, etc. But it does have GPS. Here's a blog article that describes how it works. Link
BTW, I just bought the iNavX software last night ($49.95) and it looks great. I'll try it out on the water on Friday. I'll let you know how it goes.