"...you can't activate/de-activate and re-activate the same SIM.'
That may not be quite true. Last week I did a hardware upgrade. I called Rogers to activate the new SIM that came with the phone, which, as you said, automatically de-activated the old SIM. However, right after that, even before I had a chance to place one call on the new phone, I discovered a defect with the phone. I called Rogers, and the csr advised me to return the phone to the Rogers Wireless store that I got it from, and he advised me to return everything, including the SIM card. He then re-activated my old SIM. Now, mind you, the old SIM was de-activated and re-activated within 24 hours, so perhaps you can do that if a certain length of time hasn't elapsed. That I don't know for sure.
What 99semaj said may be true too....SIM may be assigned to accounts, rather than phone #s. So if there are several phones under one account, you may be able to re-use a SIM for a different # under the same account. Again, I'm speculating.