No!
Yes it can be swapped. I actually swapped my razr's sim card to my iphone. Nothing happened. It works perfectly.
Any time you restore an iPhone you will lose your jailbreak and your unlock and you will have to start from scratch and do it all over again. You also cannot use a Boost SIM card in an iPhone as Boost is owned by Sprint/Nextel and their SIM cards use iDEN technology, not GSM. The Boost SIM cards can be used for International Roaming only and will not work on GSM networks in the US.
No
If your 3GS is locked, you can't insert a SIM card.
any AT&T phone that can take that sort of sim card.
Yes, Yes you can.
You put a sim card in it.
Yes, but only if you unlock you iphone
You can put the iPhone into DFU mode and do a full restore via iTunes, this is similar to "flashing".
As long as it is AT&T, yes.
you can take a sim from any phone and put it into an iPhone. they will all work the same, just make sure you call you phone company to let them know your going to switch phones
From your blackberry ,copy all your contacts to your sim card. Then remove the sim from your bnlackberry and put it inside the iphone. On your iphone do the import sim contacts. To give you the step by step provess on how to do the sim importing go to the link http://nice-to-know-about-iphone.blogspot.com and search for the topic "how to import sim contacts" hope it will help