Just in case anyone else has the same issue, this was my problem and solution. Had a 1st generation iPhone and upgraded to a 3GS. Ported my number to my new phone, gave the wife the old one and ported her number from her moto phone to her now "new" iPhone. She won't know the difference:)
I synced my phone and all was fine except my phone was showing off line in find my phone. All of the settings were correct but my phone was still showing off line.
I shut down the phone (hold the home button and the top button for a few seconds, then slide to turn off) and restarted the phone. Works fine now.
Just thought I'd help someone out trying to find an otherwise simple solution that I should have tried from the start.
MacPro 2.8 GHz Octo-core, iBook G4 1.33 12", Mac OS X (10.5.7)
I am having the same issue. It worked perfect on my 3G, I upgraded to the 3GS, ported the number from 3G to 3GS, now on the Find my iPhone in me.com it says iPhone 3G (not 3GS), and says offline. Let me know if you find a solution.