This information was posted on http://www.iphonedev.org/node/3 and was not written by me.
Step 1: To export your private key and certificate for safe-keeping and for enabling development on multiple systems, open up the Keychain Access Application and select the ‘Keys’ category.
Step 2: Control-Click on the private key associated with your iPhone Development Certificate and click ‘Export Items’ in the menu. The private key is identified by the iPhone Developer: public certificate that is paired with it.
Step 3: Save your key in the Personal Information Exchange (.p12) file format.
Step 4: You will be prompted to create a password which is used when you attempt to import this key on another computer.
Step 5: You can now transfer this .p12 file between systems. Double-click on the .p12 to install it on a system. You will be prompted for the password you entered in Step 4.
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