It does not answer your question of getting your private keys out of bitcoin-qt, but this link explains how you import a single private key into MultiBit:
http://multibit.org/help_importASingleKey.html
If you have more than one private key it is simply more rows in the import file.
The date alongside each key is a bit non-obvious. It has to be before the date of the first transaction using that key so that the blocks with the transactions in can be found. I recommend just finding the first transaction in your wallet and putting the date one day before it in on every row. (Using a day earlier avoids any time zone problems).
Note that the import only knows about the sipa format private keys (the ones beginning with a 5). It does not understand compressed keys. ( What is a compressed Bitcoin key? )
Unfortunately there are some
commandline geeky stuffcommands that you will need to execute. Please see my answer below. – Scott – 2013-09-14T14:18:28.5272> Some commandline geeky stuff
Well, that's the way it is done. You could pay someone to do that process for you -- but then you would need to trust them as they have your private keys. – Stephen Gornick – 2012-11-11T21:54:57.480
That "commandline geeky stuff" can be done through the debug window / console that is available with the Bitcoin.org client v0.7. – Stephen Gornick – 2012-11-11T21:56:35.963
The method of importing the wallet via Blockchain.info no longer seems to work. I can produce a JSON file from dumpwallet.py but Blockchain.info apparently fails to recognize it for some reason. – None – 2013-01-16T12:56:37.843