I wouldn't be surprised if they put a fair use clause somewhere. Otherwise you could hypothetically run a fairly resource intensive bot that scrapes data 24x7 using up aws download bw for no cost.
For example, if you start up a micro instance, you'll only be able to download at like 200kbps or something.
So I'd assume the connection speed is linked to the size of the instance, and the bandwidth cost is integrated into that cost somewhat.
wget -O /dev/null http://cachefly.cachefly.net/100mb.test