When booting my computer I am first presented with the regular lightdm login screen except there are some graphical anomalies in the top 5th of the screen. When I press a key on the keyboard the graphical the anomalies change in a similar fashion to The Matrix code. Probably the best way to explain what this looks like is having a few characters (similar to what you get when you open a binary file in a text editor but in small font) being overlaid on the login screen. When I press more keys it's as if those events are being written into the screen buffer and I'm looking directly at a memory map. This even happens when I press 'shift'.
Once I enter my password the screen flashes and I have another lightdm login screen but this time without the anomalies. On entering my password again I can login to unity and everything seems fine.
I am running the 64-bit version of Oneiric Ocelot with Nvidia 280.13 drivers. It has only started within the last week or so, which corresponds to when I was installing software to allow screen rotation and also a large set of software centre updates (including kernel update).