Verizon doesn't use SIM cards, so the Verizon iPhones presumably don't. All AT&T iPhones have a SIM card, though.
No
Iphone uses sim cards to track the user and gets all hidden information from them
No sim cards are only for numbers
Yes all iPhone have SIM cards although they are called micro sims (the gold chip only)
There are some iphones that do have dual sim cards.
Yes, iPhones use a normal sim card.
all iphones have a sim slot but on the original iphone the customer cannot easily access it whereas the 3G iphone has the sim card and a sim card ejector tool How can I access my verizon iphone 4 sim card slot that doesn't have a sim tray
The iPhone uses the micro SIM card
Yes it can be swapped. I actually swapped my razr's sim card to my iphone. Nothing happened. It works perfectly.
Buy a SIM Card from AT&T or T-Mobile called a Micro-SIM. These will fit in the iPhone. Apple is planning on using Nano-SIM Cards which are even smaller.
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.
Sprint doesn't use SIM cards...