Mario! I am going through the same exact situation you are. Read my post here:
http://discussions.apple.com/thread.jspa?threadID=1141402&tstart=0.
I'm afraid to click "Submit" because it may resign me for ANOTHER 2-year service agreement when I don't need one because I already did that when I purchased and activated the old iPhone 2 months ago. I'm just REPLACING my old iPhone with a NEW iPhone but WITH the SIM card from the OLD iPhone.
Now, ALPianoman, I read that you went through the same thing. You replaced your iPhone with a new iPhone and reactivated through iTunes and you received the same message.
I took a snapshot of the screen from iTunes and this is what it is. Is this the same screen you got?
Also, you can see that I circled in red that all current voicemail messages will be erased. Is this true even if I'm just replacing an old iPhone with a new iPhone?