I just signed up for the $50 prepaid plan and went from 4G to 2G/edge speeds after my 100meg limit. There is no 3G after you hit your quota. I did not know this when signing up and feel I was mislead.
"Some plans include specified data speeds. Where indicated, full speeds available up to data allotment; then slowed up to 2G speeds." - Right from the TOS that no one reads....I know I didn't.