An IMEI iPhone 4 will turn on if it is AT & T IMEI blocked. They provide unlock services for all iPhones. Straight Talk SIM will also work. You simply need to request for their service online from http://www.straighttalksim.com/.
As far as I know only esn's on cdma phones are blocked not imei's on gsm's. Anyway taking it to straight talk with their sim shouldn't be a problem. I'm about to try it as well. Let me know if you get it to work.