Yeah, I'm getting the same thing. Tried flashing other ROMs, but have the same outcome. Not sure what else to do, except give up and move on.
Edit: After some intensive research over the past few hours, I've found the fix! The other SBF files are not signed for the AT&T Flipout, so they fail the signature check when booting up. I found the official AT&T Flipout SBF after a while, and flashed it. It worked! My Flipout booted on the first try.
Can't post links, but google "flipout mb511 at&t firmware". Mine was the first link to come up.