I am working on a pull request to get the MultiBit encrypted wallet work into bitcoinj. It is penciled in for the next version (bitcoinj v0.7) as long as it passes all the code review and QA.
If/once it gets into bitcoinj I will produce a release candidate MultiBit using bitcoinj-0.7 with encrypted wallets for testing. Then if people are happy with it, it will go on the website. Back to one version of MultiBit to look after !
Also, I have added that when you do a private key
import it now immediately exports an encrypted copy of the new private keys into the same directory as your wallet with a timestamp. Screenshot from "Tools | Import Private Keys":

It only applies to encrypted wallets (as you need a password to encrypt the key export file).
This means that when you perform any "key sensitive" operation with an encrypted wallet it creates a timestamped, encrypted private key backup file. This is all of:
+ Add password
+ Change password
+ Create new receiving addresses (ie new private keys)
+ Import private keys.
These timestamped files are never deleted by MultiBit.