You can do what the OP proposed using MultiBit:
1) Creates a wallet which initially will have a single private key.
2) Funds the wallet's receiving address with the BTC they want to transfer.
3) Creates a private key export file with the 'Tools | Export private keys' option.
These are small files with the suffix 'key'.
You can password protect them.
4) Sends the key file to the recipient (via email or whatever) and tells them the password.
1) Imports the private key file using 'Tools | Import private keys' and enters the password.
2) MultiBit then imports the key from the file and replays the blockchain to get the key's transactions.
There are screenshots and more details here:Exporting private keysImporting private keys
PS For writers of other tools : the password protected key files can be read with the OpenSSL command
Sure, you could even achieve the same thing using the official bitcoin client, by copy-pasting wallets and creating one with the amount of bitcoins you want and sending that. The problem is that it's not very user-friendly and requires that the user actually has good understanding on what goes on "under the hood". Requiring users to create and send wallet files will also lead to many disastrous mistakes.
So I'm not really arguing for a
way to send bitcoins through other means than a regular transaction, but rather a standardized user-friendly
way to do it. The same way we have standardized QR-codes and standardized bitcoin-URLs it would be neat with a standardized file-type that "cointains" bitcoins and is recognized by any client that wants to implement it.
The arguments against using regular wallets:
1. It's unintuitive. If I send you regular cash through regular mail, I won't send it in a wallet. We should avoid confusion and let wallets be wallets.
2. Wallets are made to store bitcoins, not send them, and thus there is no standard protocol for them. Since wallets differ between different clients, you need to make sure the recipient uses the right client first.
3. It's not very user-friendly, and could possibly lead to mistakes by sending the wrong wallet etc.
The arguments against using strings of private keys as suggested in this thread:
1. Again, not user friendly. Preferably, a user should not even need to know what a private key is.
2. Exporting a private key means you need to make sure it holds the correct amount of bitcoins first. Creating a new key, sending the correct amount of bitcoins to it and then exporting it creates a lot of unnecessary steps, and possibility for mistakes.
3. Risk that you give away a private key that will recieve a transaction meant for you some time in the future. That won't happen with a standardized procedure.
Arguments for using a standardized new file type for this purpose instead.
1. Simple. It could even be a one step procedure, just enter an arbitrary bitcoin amount and click "create btc container". Everything else (creating a new private key, embedding it in the file and sending the bitcoins to the corresponding adress + any extra requirements/features for the container) could happen under the hood. To retrieve bitcoins from a container you simply open the file.
2. A file is familiar. Everyone already know how to manage, copy and share files to others. So the risk for mistakes since you don't know what you are doing is minimized. Required knowledge of cryptokeys and bitcoin wallets is zero.
3. There will be a clear distinction between wallets (which intuitively are supposed to be personal) and bitcoins meant for sharing with others.
Multibit seem to be a step in the right direction, sure. But the way I'm imagining it could be so much more user-friendly, intuitive and feature-rich. For example, by using nlocktime you could create containers that are automatically retrieved to sender if they are not redeemd before a specified time. A bitcoin client could also be created to recognize e-mail adresses in the adress field, and automatically create a container if you choose to send to a e-mail adress rather than a bitcoin adress. The most important part though, is that we have a standard between all clients, so that you don't need to worry about what client the recipient is using. The meaning of a *.btc file should be as obvious as a *.torrent file.
And as I've said, this could be huge for promotion. With a feature like this, any site with a public e-mail adress automatically accepts bitcoins in an easy and intuitive way.