I think the problem is that the MP3 file itself contains the ID3v1 data at the end of the file, and when you're streaming, that data doesn't reach winamp until the song is done playing.

Just for the heck of it, try adding an ID3v2 tag to a file, uploading that to the player, and trying to stream that one. (V2 tags are usually at/near the beginning of the file.)