News, November 2012:The problem described below went away with the upgrade to Ubuntu 12.10 (Linux kernel 3.5). Thank goodness, and high time.
There's a bug between the Thinkpad T500 and wireless n transmission.
Bug discussion here, fix here, thanks to Damon:
sudo rmmod iwlagn
sudo echo "options iwlagn 11n_disable=1" > /etc/modprobe.d/disable11n.conf
sudo modprobe iwlagn