If you’ve just replaced your old iPhone 3G with a shiny new iPhone 3GS and if you’d already set up “Find my iPhone” on the old iPhone … you may find that MobileMe keeps findingt your old phone (which, in my case, doesn’t even have a SIM in it, yet MobileMe can still put a message on the screen and make it chime).
Apple has this to say
Previously-owned iPhone/iPod touch appears at me.com/account
If your iPhone/iPod touch was configured with your MobileMe account, it may still appear when you log in to www.me.com/account, even if you no longer possess the iPhone/iPod touch. Currently, a device is removed from your account page list only after it is successfully wiped. This article will be updated as more information becomes available.
I think that means Apple knows there’s a problem but hasn’t yet figured out how to fix it.