I own an iPhone and it's a wonderful phone and surfer and everything else. But with the new iPhone and software coming down the pike [allegedly, since Apple doesn't leak its plans], I will be and others will be moving to the new phone and perhaps selling their 'old' ones on eBay or elsewhere. I had thought a hard reset would just clean it off, but NO! As with any sort of computer collector of data, there is always residual information which remains on the drive or in the memory. Macworld had a comment about this issue and an relatively easy and low-tech solution. Information on the issue can be found at Jonathan Zdziarski's site. There, he discusses the problem and his thoughts on the solution.
The original posting was at iPhoneAtlas = Refurbished iPhones