I've been wanting to jump into the Mac scene for awhile now, and these new iMacs are exactly the excuse I've been needing to force myself to do it. An iMac with a dual-core Intel chip? Man, two years ago that would have been unthinkable.
I bought a Treo 650 off of ebay to replace one that was stolen. I think it was under $100 at the time and was at the end of its run as a current smart phone. You might not have great luck since you are looking for a $600 phone without a subsidy.