Short answer, no - (for the prepaid telephone calling card) it is not recommended. You still need to use "local" but roaming airtime to dial into network. They are designed for pay phone (or local landlines.) If you are asking about the prepaid Sim card that you insert into your unlocked iphone with GSM bands, yes.

Are you visiting BDA briefly by air or by sea? Do you plan to visit & stay on a regular basis and for how long? If this is only for a very short stay, emergencies or those just in touch situation - it's sometimes better to just enable international calling from your home carrier (AT&T or T-Mobile, etc.) and pay the overseas/long distance rate - instead of going thru all the trouble. Also, do you need or plan to use broadband data on your iPhone 4 or is this for voice/SMS only?

Your best option would be to have your iPhone4 unlocked, and then go to one of the local Bermuda cellular store (there aren't many, if any, independent agents & newstand/stores don't "stock" different prepaid Sim cards & refills, unlike the US) to get a compatible card and activate it, add money for airtime (and, if needed, data usage) and you will be set to go - to call the US and to receive incoming calls on a local BDA number.

More specifics, please. Not knowing more about your usage on the island, that's about all I would advise.