As blogged on raspberrypi.org, the USB fuses are no longer on some of the later revision 1 PCBs or the revision 2 PCBs
USB Output Power
The resetable fuses protecting the USB outputs have been removed. This feature was implemented on some later revision 1.0 PCBs by replacing the fuses with links; revision 2.0 permanently implements this modification. It is now possible to reliably power the RPI from a USB hub that back feeds power, but it is important that the chosen hub cannot supply more than 2.5A under fault conditions.
Is is relatively safe to bypass them on my Raspberry Pi so I can use a wider range of keyboards/wifi/harddrive with less problems?