With the most recent update, you can attach money to an email just like you would a file. For record-keeping purposes, the workflow is beautiful: you can respond to an email request for money with another email containing the money.
Transfers from a checking account are totally free, while transfers from a credit card are charged $0.30 or a 2.9% fee, whichever's greater. If you are a Google Apps user (i.e. your address is @somewhere-other-than-gmail.com), your organization's administrator controls whether you can sign up for Google Wallet with that address. This feature is only available to US residents.
Like most new Google features, this one is being rolled out slowly. If you can't seem to attach money in Gmail yet, shoot me an email and I'll send you $0.01 — an hour or so after signing up for Google Wallet and accepting the transfer, refresh Gmail and the 'Attach Money' option should appear.
