Currently you can indeed import/export public keys with the Free version. Not sure how long we’ll retain that functionality. Not because we want to squeeze folks for their money, but because it offers too many ways to get into trouble. These features are there right now mostly because they were used during development. Most likely we’ll retain them but in an ‘Advanced’ mode preceded by various disclaimers and mostly for situations where Internet access is not possible at all.
With Premium, you obtain the public key using the recipient’s e-mail address and the simple “Key Sharing” dialog. Pretty foolproof and simple, is the idea. Also, with this feature you can actually do this before the recipient has even signed up, as we’ll generate a key pair and encrypt it temporarily with a server-based machine-key until the recipient completes the signup process and sets his or her personal password.