Sometime, depending of the wireless access point used, my wireless connection looks stuck on my Ubuntu (11.10 or 12.04): nothing special on the network icon, but no more connection, no ping, etc.
The problem comes from the the 11n support which is not well supported.
The workaround is to turn off the 11n support. To do so, edit
/etc/modprobe.d/iwlwifi.conf and add:
options iwlwifi 11n_disable=1
It works like a charm.