I'd stick with the iMac until it either broke down or no longer served my needs. I'd also look into lengthening its shelf life with either a ram or hd upgrade (which looks to be a b$#(h from the guides I've seen online) before I got a new system.
I'm chugging along with a white c2d 20" imac for my office, hooked up to a second monitor, and I honestly have no desire to shell out the money for a new system.
I wouldn't consider myself a mac oldtimer by any means, but I did come on board during the last few years of the PPC era. Besides the security and stability of the OS, what attracted me to the platform was the number of guys I knew using macs to make a (non graphic or video arts) living who were contentedly working away on two or three year old machines. This was due partially to the ppc upgrade cycle (why spend thousands on a new machine with 500 extra mhz) which contrasted nicely to the newer, better, faster upgrade every 6-18 months pc mentality, complete with windows revisions that forced you into a new machine.
Unless you have scads of disposable income that you want to throw into a new machine (and God bless you on behalf of the economy) or have a job where you're losing potential income with every processor cycle, why switch?