My brother has the same problem with his K8N neo2 platinum, and from scouring the internet i have found it is a very common problems with these models
Not worth RMAing it, as it would probably cost you a tenner (per motherboard) to do that, your better off just buying a PCI USB2 card with an extra connection to plug your front USB's into.
Some of his USB sockets do work o.k., but the others turn on and off and you find you have to move them to another socket to get them to work, but then next time you turn on the pc it isnt working again
Or you could try a bios upgrade that might help
(System cannot resume from S3 sometimes when install USB device.)
http://www.msicomputer.com/support/bios_result.asp?platform=AMD&model=K8N%20Neo-FSR/Platinum%20(MS-7030)&newsearch=1