During an iTunes sync, are MacBooks prevented from going into sleep mode? I want to know this as I sometimes leave my iPhone unattended while syncing and I want to make sure that there's no cut off to the iPhone sync.
Your MacBook shouldn't go to sleep while iPhone sync is in progress.  iTunes' sync needs to copy files from your Mac to the iOS device, and vice-versa. In either case, your Mac is accessing the hard drive.

Refer to Mac OS X: Why your Mac might not sleep or stay in sleep mode &ndash; Apple Support. This section roughly halfway down the page is relevant:


  Open applications can affect sleep
  
  Software that accesses the hard drive will delay sleep mode until it
  is either quit or done accessing the drive.  For example:
  
  
  Songs playing in iTunes.
  A movie disc playing in DVD Player.
  
  
  [... lots more detail in the article ...]


In your case, songs aren't playing in iTunes, but iTunes does keep accessing the drive as long as the sync is in progress, which should keep your MacBook awake.