Version 0.6 of the Satoshi client will have the exportprivkey RPC call, which can be used to extract the private key corresponding to an address. This should allow you to import it, but it requires probably too much manual work to retrieve all keys one by one. Also, you'd need to scan all transactions in the wallet to find out where they were first used.
Additionally, my showwallet branch
allows exporting the entire wallet (organised in information per key, and optionally which blocks and/or transactions they were used in, in a human-readable JSON file format. It still needs a bit of work, but I hope to get this merged soon as well.
Finally, note that I recently sent a mail
to the bitcoin-development mailing list about an extension to the base58 private key format to indicate compressed public key addresses, which will also be used as of the 0.6 release.