BES will push email to a device regardless of the carrier network it's on. I wasn't aware that Cingular had its own BES and hosting is definitely not automatically included with their data plans. Please note that your carrier's data plan and BES hosting are two separate things. I personally wouldn't trust Cingular with BES hosting...

Your real issue is that I doubt you'll find a carrier that offers Blackberry data on a prepaid SIM. You must have a Blackberry data plan to receive email.