I have both a Windows laptop and my MacBook Pro. Right now, I am still synching with my Windows laptop until I have everything transferred to the MBP. You won't be able to sync contacts, etc. with more than one computer, and the MacBook won't use the iPhone contacts to "set up" a new contact list, instead it would overwrite the iPhone with a blank contact list if enabled.
I would suggest using some third party tool to move your contacts to that will sync with both your Windows PC and the MacBook, and then you can switch. (At least that's my plan.) Moving photos, music, etc. is much easier, other than my playlists didn't migrate...
