(I'm a noob so I can't bump old threads, but I can at least create new ones)
MP3 is still the format which reaches the greatest audience in terms of support (native where possible or otherwise Flash).
Talking about greatest audience, I noticed you can't play musics from chipmusic.org when you're on an android smartphone and tablet, and I guess it's the same on iphone/ipad because they no support the flash player used on this website. I think it could be relevant to switch to an html5 player such as this one: http://pupunzi.open-lab.com/mb-jquery-c dioplayer/ (it can also use a flash player when it's not supported)
And being able to import flac or ogg tunes would be great too (even if they are converted to mp3 after)