I kept my original iPhone for a WiFi device with camera, when I went to the 3GS for a phone. I gave the 3GS to a neighbor when I went to the iPhone 4S, but I still kept the original iPhone.
Mostly I use the older device as a backup cam in my studio in case my 4S is charging or just not in the room. I often snap a picture of something I'm working on, or document the available precut sizes of some fabric I have just carved up (so I won't be thinking I have 3/4 yard of something when it has just become a bunch of 6-1/2" squares or 4" strips or whatever). It's really handy because I don't have to make any notes or really even stop working, just snap the photo and then when I import the cam roll later, I can decide how I need to proliferate the information or further annotate it.
It's really important to my workflows to have the photos I take during the day in the studio, but I've found if I don't keep a backup cam of some sort in there, I'm tempted to skip some documentation steps that I really need to complete. So my original iPhone is a treasured piece of gear. I sure will miss it when it croaks. I hope Apple is still making a an iPod touch with a camera at that point because using an big ol' iPad-x for this backup cam function would feel like real work, compared to one-handing a shot with a phone-sized device. Maybe I will have to keep my 4S instead of handing it on to a neighbor when I want to upgrade again...