I just ran sudo apt-get update. I'm not sure if this updated bitcoin-qt or not. bitcoin-qt is still working.
apt-get update just updates the list of packages, you need apt-get upgrade to actually upgrade packages.
How can I see what version of bitcoin-qt I currently have?
There should be an "About Bitcoin" button under help in the Bitcoin-Qt window.
I heard that on May 15th, we will need the newest version of bitcoin-qt. Is this true, or will older versions still be OK?
Yes, you need to upgrade to at least 0.8.1 before May 15th (0.8.2 is also really nice, so upgrade to that when its out). That said, its generally a very, very good idea to always update both Bitcoin and all your system packages very regularly, both because of new features and options and because the Bitcoin network benefits greatly when users upgrade.
how do i check the version from the CLI?