Not that simple...
At least not with MP3s. Because of the encode/decode and the frame-length requirements, MP3s are hard to degap without extra information from the encoder (iTunes and LAME both add this information, in different proprietary ways).
xmms-crossfade cheats by buffering up the next song and figuring out the gap by first principles, apparently.
Regards,
-scott anderson
"Welcome to Rivendell, Mr. Anderson..."