Today is a fairly momentus day, in that 3 releases of Ubuntu all reach their end-of-life today. From today onward Ubuntu 8.04, 10.04 and 11.10 no longer receive security updates.
If you are still using one of these versions of Ubuntu, I highly recommend you upgrade to at least 12.04. If you're using Ubuntu on your desktop, I see no reason why you don't upgrade all the way up to 13.04. Servers should stick to the LTS releases though.