Okay, I've flashed to OS2008 and I get the blue progress bar. I think that there was one occasion that it might not have come up, but I can't confirm that.
I'm haven't charged it while it is on to avoid the power-on-problem. But could it be the possibility that the tablet doesn't turn completely off when plugged into the charger that was brought up power-on-problem threads that explains the lack of a progress bar?
Just a guess...