Comment 79 for bug 126369

Solved by a Lenovo BIOS upgrade which is now available for X61t machines as well. See here: http://www-307.ibm.com/pc/support/site.wss/document.do?sitestyle=lenovo&lndocid=MIGR-68005
The following changelog line is relevant to this bug:

Version 7SET23WW (1.09)
    * (Fix) Unexpected interrupts from the USB controller may occur. (BIOS)