There is a very useful utility called security(1) in OS X, which lets you manipulate your keychain from the command line. You can easily im- and export keys and certificates using this. This is especially useful for AppStore developers, who code on multiple Macs. Having the signing keys in sync is kind of a challenging solution, if you don’t use keychain syncing via MobileMe. For example you can import a key like this into your login keychain:
$ security list-keychains
$ security import -k /Users/yourguy/Library/Keychains/login.keychain somekey.pem
1 key imported.
Hope this helps.