We recently encountered a problem with our Ubuntu servers were they would boot up to the login screen, and continuously display an error where tg3_stop_block timed out.  After a bit of searching I found that tg3 stood for tigon3, and was the nic driver.  This wasn't an issue in 11.04 (11.10 on the servers), so I checked the driver versions for tigon3 between Ubuntu 11.04 and 11.10, and 11.10 had a slightly newer version.  Also the tigon3 driver was inherently part of the kernel, and broadcom wasn't offering any sources/ debs.  So a rollback was out of the question.

I continued to research this, but didn't have much luck diagnosing the problem.  So eventually we just decided to see if it was an issue in the 12.04 beta.  Oddly enough during the first install of 12.04 the installer prompted me for a tigon3 driver, so I looked around and found one on the dabien website.  I have a feeling the driver I found was old/ not for Ubuntu, because when I tried feeding it in, my install corrupted and I had to start over.  On the second attempt I skipped the missing drivers, and it installed flawlessly.  The stop_block error was also gone. When I checked, the tg3 version was newer, so it seems the issue was fixed between 11.10 and 12.04.

Update: 07/02/2012
Turns out the solution was kernel related... Ubuntu 12.04 comes with a stock 3.0.23 kernel and herein lies the problem. I explained the details on the askubuntu.com stackexchange on how to fix the problem (hint - just upgrade your kernel (apt-get dist-upgrade))