According to this there is a report of the similar kernel version as used by oneiric having the same issue...
It is probably worth experimenting with different kernels to see if this resolves your issue - see the possible solutions below.
You should file a kernel launchpad bug report so that the Canonical Kernel maintainers are aware of the issue - possibly they may backport a patch from the latest release candidate in a future kernel build.
When filing the bug-report, they will ask you to test the latest mainline kernel. To do this, following the instructions below.
to install a kernel
Navigate to this website and download the files that look like the following:
headers_amd64.deb, headers_all.deb & image_amd64.deb if using 64bit Ubuntu
headers_i386.deb, headers_all.deb & image_i386.deb if using 32bit Ubuntu
Install the packages downloaded using the terminal command:
sudo dpkg -i linux*.deb
When booting press SHIFT to display your GRUB - this screen lists your kernels - you can boot with any kernel listed including "older linux" option for old kernels.
Look at the kernel mainline and choose a newer kernel - preferably one that has the oneiric name associated with it - however you could also try the very latest (but probably unstable) 2.6.32 release candidates.
Looking closely at that bug-report, early in the thread the OP reported that his issue was due to a hidden SSID. Possibly you can check in your router settings whether you are using a visible SSID or a hidden SSID.
Also, the OP reported that he didn't have his particular issue with the older 220.127.116.11 kernel. You could also try that by downloading this from the launchpad mainline and installing this using the instructions above. Remember to booting into the older kernel by selecting it from GRUB.
If this doesn't workout for you - remove this kernel using my answer in this question
If the newer/older kernel works for you then you can use my answer here to "fix" the kernel i.e. to always boot into the newer/older kernel.
As we have discussed - trying all of the above, booting with the older 18.104.22.168 kernel seems to provide you with the best solution.