ok, i have a sony ericsson W800i and it happy to play the (unlocked) .m4a files from iTunes, which i'm quite happy about, but it doesn't display the tag information (although it is present according to Tag&Rename and the likes)

is there ANY way to get the W800 to display the tag information? or are mp4 tags non transferable to ID3?

mpeg-4 AAC tags are in a different form than ID3 tags that are associated with mp3's. I noticed that my PSP played the few mpeg-4 AAC files that I have yet it would not display the tag information. I am not sure what format the tag needs to be. You might try opening the song/s in the Sony Connect software then editing the tags via that. It worked for my PSP so it will probably work for your Sony DAP.