Just saw http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=b7798d28ec15d20fd34b70fa57eb13f0cf6d1ecd - I've got the same problem with my new E6420.. See https://bugs.launchpad.net/ubuntu/+source/linux/+bug/780757 for more details.
Fixed in http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=a536877e77f73ea22d12d94a019fedd9671b6acd