Apple does some things well, but it has its share of persistent defects and annoyances.
A search of Apple's kb on "user switching" iPod iTunes doesn't find any articles as of 5/07, but anyone who's switched users (accounts) on a machine with a connected iPod knows bad things can happen. The iPod is bound to a user account, not to a machine. When a new user takes over, the connected devices is passed to the new user -- and the OS offers to fix the corrupted device.
This is a non-trivial problem to fully solve, but Apple could have done a lot to mitigate it. The OS should suggest dismounting connected iPods on logout or switch, and iTunes should be smarter about how it responds to a connected iPod post switch. Apple hasn't fixed this because only a small minority of their customers have multiple users on a single machine. In other words, they don't have enough family customers.
Update 9/9/08: iPhone/iTouch 2.0 finally fixed this problem! They are fast-user switching safe on OS X.