Comment 31 for bug 574307

Fiepel, the WLAN not reconnecting on wake up is another known bug (most likely #661206)

The orinal submited bug is a real one involving the rt2860 driver.
With Ubuntu 9.10 kernels, no problem.
With Ubuntu 10.04 kernel, you'll have to reinstall rt2860 driver as described here : http://ubuntuforums.org/showthread.php?t=1476007
With Ubuntu 10.10, the driver for rt2860 cards is different (if you're an advanced user, you'll notice some strange values, and the iface name has changed) but it is working.

I have this problem only for mixed WPA/WPA2 connections