The rtl8187 driver in 2.6.31-rcX will not associate if the device is
plugged in at boot time. Unloading/reloading the driver does not help,
but unplugging/replugging the device does. It also works if it is not
inserted until the system is running. External devices can be made to
work, but not internal ones.

The patch that fixes this problem was submitted yesterday (8/20) and
it should be available in kernel 2.6.31-rc7.