Microsoft’s Windows Media Player 10/11 can also write ratings to tags, but it’s “off” by default. Most applications just maintain ratings in their own database, like iTunes.
Ron will need to comment about the conversion. iTunes’ UI only lets you set “0,1,2,3,4,5 stars” (even though the UI can display half stars)… internally I think it’s 0-100, and the reason Firefly uses that scale. WMP also sticks to the full 0-5 star model in the UI.
Remember, there is no single “correct scale”… personally, I think 255 values is a bit overkill. 😀