Almost all modern players (be they Windows, Mac, or portable) will handle M4A/AAC encoded files. It has become the industry standard in terms of replacing mp3. If you download songs from iTunes, you are already playing back m4a files – you just may not realize it. It is the same format Apple uses for iTunes.
There are a number of reasons why it is a challenge to encode mp3’s on an iPad or iPhone. As a result, the short answer is that it’s not likely that we’ll be adding an mp3 export option to Anytune itself. If for some reason your device requires mp3 instead of M4A/AAC, then there are a number of options available on the desktop to convert between the formats. iTunes even has a converter built in.