I don't bother with screen protectors since my DS. Haven't managed to scratch my Razr, G1, G2, or any other glass-fronted device.. then again, I'm ridiculously careful to make sure if I'm wearing rings that they never touch the glass, that the device doesn't go into a pocket with anything harder than a receipt in it, and that I don't set anything down on top of it even 'just for a second'.
Only time I get any kind of difficulty sliding on the glass is playing games like PewPew or AirAssault for more than twenty or thirty minutes at a time... only up side to a screen protector might be a slightly more 'paperlike' feel if you're using a stylus.
Oh, and I'm not using a carrying case, slipcover or anything like that.
Really don't understand how people can scratch up their devices, taking even basic precautions. Don't let the dog chew on it, don't hand it to anyone in the single-digit age bracket under any circumstances, don't stuff it into a pocket with your keys or loose change, be aware of what's on your hands and where they are.