The 191MB v1.2.1 update is available on FileShack, with the German version once again getting a separate update due to its censored nature. Please note, these are only for retail copies--if you bought Borderlands PC through a digital distribution outlet, you'll need to get the patch from them.
As for patch notes, here's what Gearbox had to say:
The update contains the following:
- Multiplayer connectivity has been improved; users should no longer be required to forward ports to host or join multiplayer games
- Players who experienced distorted shadows or graphical glitches after update 1.0.1 should no longer experience these issues
- The 3D Vision crosshair has been improved