This was taken from a very helpful article over at skabber.com. I develop on a lot of machines and transferring Apple developer certificates is an absolute pain! — Thanks Jay! You saved me heaps of time!
If you ever need to install a certificate into the OS X Keychain on a remote machine in such a way that Xcode can sign a build with it, then this command is for you.
security import /tmp/MyCertificates.p12 -P MyPassword
This allows you to not get that annoying “Always Allow” dialog the first time you try to use the cert to sign a build.