IPhone 4 error message? The sync session failed to start?

The error says, "The iPhone 'Molly's iPhone' could not be synced because the sync session failed to start." I have an iPhone 4 and it has 4.2 GB of free space. I have a macbook pro and iTunes 10.7 (it's fully up to date). Please help I'm due for an update in 2 weeks, but want to put my new music on my phone.

Unplug it and plug it back in
i have problems with my ipod where it said that too an i unplugged it til it work

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