I'll see if the private key is present on the disk image at all after 0.3.24->0.4.1, 0.4->0.4.1, and ->0.5 when converting and encrypting (which requires care to not inadvertently put the key on the disk by searching -> MUI registry entries, save keys in text files, pywallet --web -> browser cache, etc).
Absolutely no guarantee is made that old, pre-rc6 private keys will not end up unencrypted somewhere on the disk.
There is no guarantee that newly generated, post-rc6 private keys will not end up on the disk, either, although the code tries to keep that from happening (locking memory so it is not swapped to disk, for example).
There should be no files containing unencrypted private keys after rc6 rewrites the wallet, though.
Thanks for helping test!