Iphone newbie questions/concerns?

[April Ryan]

As a lifelong Blackberry user, I have absolutely no knowledge on the Iphone. I want to make the switch in the next day or two, but have a few questions, and was hoping an Iphone veteran can help me out:

1- If I buy an Iphone and it is not unlocked/jailbroken, is there a certain firmware the phone must be running for me to do it myself? (will be either a 3gs or 4 btw)

2- Other than receiving EDGE instead of 3g speeds, if I stick with Tmobile will there be any other features or applications that do not work correctly?

3- My Tmo contract ran out and to save a few bucks for the holidays, I switched to Tmo's prepaid plan. I have fallen in love with the price of this plan, so can I use this prepaid plan (it includes data) with the Iphone, or do I need to switch to a regular smartphone data plan? This current prepaid plan has been giving me the same 'smartphone data' on an HD2 that the 'contract smartphone data' did, so I was just curious.

4- I'd like to know what jailbreaking does in terms of apps. People say you get free apps- does that mean ALL of the apps are free, or just a few here and there?

5- For anyone who has typed on either a BB 9550/9530 or HTC HD2/HD7, can you compare typing on an Iphone vs one of those?

6- For anyone who has owned both the 3gs and 4, ASIDE from cameras/flash/screen res/design, what are the pros and cons?

7- Just how fragile is the glass? Will one drop crack it, or is it fairly durable? (I drop my phones alot and hate cases)

8- If I buy a used one off Craigslist, is there anything I must be aware of when checking it out (other than the usual call test/screen check stuff)?

Thank you to anyone who can give me some insight, feel free to add additional (constructive) info I should know as well- I really appreciate any help I can get
To responder #1-- LOL, wow. Note I said "IF" I stick with Tmobile. I stated my Tmo contract was up, so a simple 'hey, it'd really be better for you to just switch to ATT" would've sufficed. That was mainly what I was looking for.
To responder #2-- Thank you so very much for taking the time to help me out.