I have had this issue with 10.04, 10.10, but I was able to solve it by custom compiling the driver from recent sources. And I had to re-compile, very painfully, whenever there was a kernel update.
And both versions of ubuntu did not have the ath9k driver by default. But with 11.04 it looks like nothing can be made to work. Natty installed the driver by deafult, and it froze my system when I turned on the wireless. I compiled and installed from the recent versions, still the system froze. I installed the .deb I created with 10.10 (which had worked there), again the system froze.
I again compiled from the older sources from which the 10.10 working .deb was created, again the system froze. I usually put my netbook on master mode (as an access point) to share the internet connection my family members. From now on, untill a solution is found, we all have to fight for the single ethernet cable to the modem.
Edit1: I directly logged in to CLI, without logging in to GUI. In the CLI, when the system froze, "Kernerl Panic" was shown, and near it there were ath9k errors too. However, the error did not get logged at
EDIT2: Same issue with Ubuntu 11.10 too. The moment eth0 and wlan0 are bridged (with hostapd) the system freezes. And the freezing happens only if I enable wireless master mode. Even after googling for hours, I was not able to solve the issue.