Yes you can use Straight talk on your iPhones. The sim card costs $14.99 and can only be ordered online. I ordered an AT&T compatable phone SIM card. You do not need to buy or use Nokia N71 card.
I have been using their service for the past two months on a 3gs phone. Locked or offically unlocked did not matter. The phone is NOT jailbroke either. The only three issues with ST that I am aware of are: no MMS on messages, no hotspot/tethering, and no visual voicemail. You do get notified of voicemail with badges and in the notification windows. The only set up required is to take safari and point to http://unlockit.co.nz to build a network configuraion file to enable data. (Voice and text work without the config file.) This site uploads a configruation file that is the same as one built by the Apple Configuration Program. The site is FREE and provides help. It does not "Unlock" your phone like AT&Tcarrier unlock or other jailbrake programs. $45 a month for unlimited talk, text, and web. I do not use international calling, but would consider skype or google voice for that. I have averaged 2.6gb of data a month with no cap on speed either.
Site with more information:
http://www.mymoneyblog.com/straight-talk-sim-card-iphone-4s-45-unlimited-prepaid -plan.html
http://www.straighttalksim.com/