OK, first the bad news. Your dealer may be correct about the engine ECU being the problem. The good news is that there's a simple voltage test that they can do to be sure. Keep in mind that the no-start must occur while the test is being performed.

Ask them to look at document LI54.21-P-052569. It concerns hot-start issues on M272 equipped vehicle produced from 11/2006 thru 5/2007. It's possible they've already done so, but it can't hurt to ask. In my experience, it's a rare failure. I've only see it happen a couple of times but it's worth looking into. Good luck and let us know what happens.