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.
<note from 1st September 2009: this problem is now fixed. MobileMe now seems to know about both my old iPhone and my current one. Nice>