Most of the delay in my opinion has to do with the music companies not wanting to give the same broad rights to the Windows side as they did to the Mac side. Its rather obvious that we only got these great rights on the iTunes Music Store because we are just a small slice of the computing world, and we are basically guinea pigs to this whole type of business model.
I am rather confident that Apple will succeeded on the Windows side because most of the complaints about BuyMusic.com are due to its bad service and user experience. Two things that Apple has nailed with their service.