Lets start out with the iPhone 4 & the iPad have VERY good gps's in them. They do not need a cell signal to work. They have AGps chips, or assisted GPS chips, they use the cell towers to quickly (a few seconds) lock on to where you are and then use GPS satellites to further refine your location. If there is no cell signal they will just use the satellites and take longer (20, 30 seconds) to acquire a your location.
I do not use my dedicated out door gps any more. I use my iPhone for cache's and my iPad when I hike with my back pack.
They work awesome. If your geocaching you might want to look into get a new iPod touch and a gps add on for it
there are 2 that I know of
http://www1.magellangps.com/toughcase/ (the one out now is for iPod touch 3rd gen, 4Gen version should be out soon)
Cool thing is once you add one of these they will work with ALL GPS IOS apps!
The geocaching.com app is probably the best Mobile geocaching app ever. Also there are some very slick GPS hiking apps with topo maps:
Gaia GPS http://www.gaiagps.com/ and accutera http://www.mobile.accuterra.com/ Plus a tons more
I would say it makes sense to get something like this over a dedicated GPS now