“Sync multiple iTunes music libraries with one iPhone” by mrmobius of Mac OS X Hints discusses the steps necessary to synchronize songs from multiple iTunes libraries to one iPhone.
Tip author mrmobius describes how he puts files from two different libraries on his iPhone. He keeps his MP3 library separate from his collection of high quality (lossless) audio, but puts select tracks from both collections onto his mobile.
Mrmobius notes that this tip could theoretically be used to sync as many libraries as you want. I also imagine that it works with any iPod and not just the iPhone.
Photo by iLounge.
