its called mobile me and i have it set up on my daughters iphone 4 (its brilliant) but it doesn't work on mine, the 3g says not compatible :-(
Bit of a trick for you:
You're right MobileMe won't work on the 3G, UNLESS you set up the account on either an iPad or iPhone 4 FIRST.
Once the account is active just enter the details into the MobileMe on your 3G and it doesn't authenticate the phone so gets past the restriction.
100% works as I have several linked to my account, can even do it with the early iTouch and it transmits when on WiFi.
I did this within a few days of it being released and even let Apple know as THEY said it wasn't 3G compatible but they didn't seem too interested.