Are you the only one on your account, or do you have a Family Plan? If you're on a family plan, check to see if any of the other lines are Upgrade Eligible, you can easily use their eligibility to get the fully subsidized price. Otherwise, you can do as you said and add the line, then pay the ETF after 30 days or whatever it is. Just be sure to restore your phone before giving it to your co-worker. And as far as the SIM cards go, just take your card out and let them put their card in and things should be just fine. Then when you get the 3GS it'll be ready to put your SIM card into and you'll be off and running!
Hope that helps a little.