Bounty for the 50 coins. The first person to get a patch merged by Gavin into the core software that allows import/export of wallet files, via the GUI on all 3 supported platforms, defined in the following manner wins the coins. Obviously not very much but I guess it's symbolic 
The format should be a CSV file (unix line endings) that looks like this:
where the block numbers are the blocks in which there are unspent outputs sending to that key.
The format should be a CSV file (unix line endings) that looks like this:
Code:
base58 encoded privkey,block number,block number....
base58 encoded privkey,block number,block number....
base58 encoded privkey,block number,block number....
base58 encoded privkey,block number,block number....
base58 encoded privkey,block number,block number....
where the block numbers are the blocks in which there are unspent outputs sending to that key.
CSV file with the private key and block numbers is a good idea, although for it to be a valid CSV file then it needs to have a fixed number of columns.
I'd modify the design slightly to be just:
base58 encoded privkey,block number
... where block number is the block number of the earliest input (that'll save rescanning time-- you probably always want to rescan from the earliest block number, anyway, in case more payments were sent after you exported the key).
Also what do you mean by "export" -- write and then remove the keys from the wallet? Write a newly generated key and generate a payment-to-that-key for a given amount of coins?
I think any code that removes keys from the wallet (or generates payments to keys that are never added to the wallet) needs to be structured as two distinct steps:
1. Write the keys to <destination>
2. Read <destination> to make sure it is valid, and, if it is, delete the corresponding keys from the wallet (or generate the send-to-self txn).