Let the hive mind of Engadget get that for you.
"I just switched to Sprint from Verizon about three months ago for the Pre. Then I went for the Hero about a week ago. Now, I miss my hardware keyboard and am thinking about switching to the Moment. I am still able to switch back to Verizon if I want and get the Droid when it arrives. Should I just trade up to the Moment when it comes out, see if I like it, and if not switch to the Droid? Or something else entirely? Help!"
an iPhone and Blackberry SIM swapping scenario isn't going to work too hot with the iPhone SIM. For a Blackberry to function, the SIM has to be placed on a Blackberry Plan. This makes it so it actually runs on a different network segment than typical phones. So you can use a Blackberry SIM in a regular phone, but not the other way around. I have my Windows Mobile phone registered on the Blackberry network so I can use ActiveSync or Blackberry connect. The iPhone on the other hand complicates things a bit. An iPhone won't work with a SIM unless it's been activated in the phone, or you Jailbreak it. So the only shot at using the same SIM in both phones is with a Blackberry enabled SIM. The only shot of using a Blackberry enabled SIM in an iPhone is by Jailbreaking it.