PUBLIC MARKS from multilinko with tags digital & store

Buying and using digital music is a far from easy decision today - there are various competing and incompatible formats, stores and players out there in the market, primarily Apple (AAC + iTunes + iPod), Windows (WMA + various stores + WMA-compatible players), and Sony (Atrac3 + Connect.com + Walkman). How do you then ensure that the music and player you buy today will not be incompatible with your player, online store or the OS?