There is a comprehensive explanation of the remaining USB issues and limitations at:
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=23544&start=324
and also an explanation of USB serial port limitations at:
http://www.raspberrypi.org/phpBB3//viewtopic.php?f=29&t=42586
FIFO issues:
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=39175&start=13
high bandwith Isoc:
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=39175&start=37