I haven't been able to find anything definitive here, and the question comes up regularly, so I did two experiments... Turns out it's scary easy to move iTunes from PC to Mac and preserve all the playcounts, ratings, volume adjustments, playlists, etc.

I tested on XP/SP2 and OSX 10.3.9 using current iTunes and QuickTime. The PC had fresh installs for both tests (under the owner account); the Mac had new users (standard, no limits) created for each test. The PC iTunes tracks had playcounts, volume adjustments, ratings, playlists, and start/stop times. The song files included a complete album, tracks from single-artist albums, and tracks from compilation albums; half the tracks had album art. Each computer had an external HD. Files were transferred with a FAT32 drive.

Test #1 - Default iTunes Music folder location
Starting with a "stock" iTunes on the PC, I copied the entire contents of the "iTunes" folder from:
\Documents and Settings\username\My Documents\My Music\iTunes\
This included the two library files ("iTunes Library.itl" and "iTunes Music Library.xml"), a folder named "Album Artwork", and the "iTunes Music" folder. Then I said farewell to the PC...

On the Mac, I fired up iTunes once, just to make sure its prefs file and storage folders got created. Next I quit iTunes, and deleted the "iTunes Library" file from the /Users/username/Music/iTunes/ folder. (You can delete everything in that folder if you wish, but at least delete the iTunes Library file.) Then I copied over the four PC iTunes items -- two library files, two folders -- into the Mac's "iTunes" folder. Then I fired up iTunes...and everything was just as it should be. All the tunes were there, all the playcounts, all the ratings, all the playlists, etc. (Feeling brave, I then quit iTunes, located the "iTunes Library.itl" file I'd copied over from the PC, and removed the ".itl" extension from the filename. Fired up iTunes again, and everything was still fine.)

Test #2 - Custom iTunes Music folder location
This test started with an iTunes Music folder named "miTunes" located on the external drive (F:\miTunes\). I made a copy of that music folder, plus I copied the entire contents of the "iTunes" folder (just as in Test #1).

On the Mac, before I even opened iTunes, I copied the PC's music folder to the Mac's external drive (/Volumes/Toonz/miTunes/). Next I opened iTunes, and set the Prefs>Advanced>General>iTunes Music folder location pref to the "miTunes" folder. Then I quit iTunes, and deleted the "iTunes Library" file from the /Users/username/Music/iTunes/ folder. (You can delete everything in that folder if you wish, but at least delete the iTunes Library file.) Then I copied over the four PC iTunes items -- two library files, two folders -- into the Mac's "iTunes" folder. Then I fired up iTunes, and again everything was right as rain.

So there you have it: switching iTunes is so easy that...even I can do it ;-)

S2