It may have happened in a later iTunes 9 update, but iTunes 10 definitely includes a very helpful update. Plug in any iOS device, and you can play its music or videos without having to make a tricky sync.
iTunes' habit of insisting that every new iPod or iPhone plugged in must be a new purchase that you want to sync to your Library — and therefore wipe out all media on the device — was among the worst of its habits. In iTunes 10, and perhaps some up-to-date iTunes 9 versions, you can now plug in an iOS device, tell iTunes to cancel the sync request (and check off "Don't ask again"), and from there on out simply play music or movies off whatever device you have plugged in through that iconic white cable.
The second-look discovery was made by the 9 to 5 Mac blog, and its commenters have a few tips and clarifications on iOS/iTunes syncing at the full post. If you're looking to do more than stream tunes, read our guide to copying music from your iPhone or iPod to your computer for free.
Little known iTunes 10 benefit… [9 to 5 Mac]