Sorry for bumping but I thought I'd chime in since I was doing a bit of research to find out whether I could use my iPhone 4 to place caches.
Personally, I can't compare to any sort of earlier experience with dedicated GPS units, or with a 3GS, since the iPhone 4 is both my first smartphone AND GPS device. I bought it at the Canadian release date (in July). It's actually the iPhone 4 that brought me to geocaching, because I'd heard about it in the media before and figured I'd give it a shot with my iPhone. I had no idea what a good coverage was, but it did (and still does) get me to the caches I'm looking for most of the time with an accuracy between 3 and 8 meters depending on weather and surroundings (it's slightly less effective in the city and under trees). I've never had experiences where the phone brought me very far off the track. What I've found out though is that it's a good idea to wait 20-30 seconds after opening Google Maps to make sure the phone locks the position correctly. From what I've read, it seems the iPhone's advantage over regular dedicated GPS devices is that it uses cellular signal to give a rough approximation of the position, which makes satellite positioning a lot faster so that explains the comments about the incredible speed.
I bought a cheap used Garmin Geko during the fall because I was enjoying geocaching a lot and wanted a device that I would not worrry about dropping/getting wet/breaking. I guess it's not the best device to compare the iPhone against, but the few experiences I've had with it were pretty unsatisfying (things like the Geko telling me I was spot on and the iPhone telling me I was 100m west and the iPhone being right), so while there are reasons one might want a dedicated GPS device (like durability, and being able to whip the device out anywhere on Earth without being afraid of having a dangerously high cell phone bill), I don't think the dedicated device vs. smartphone fight is justified, at least in terms of precision.
Mind you, I will probably triple check my caches if I do decide to use my iPhone to place them, just in case.
