Try another SD card and see how that goes. And if you've got a Linux box with an SD reader, you could plug the first card into it and see if there's anything interesting in /var/log/syslog. After what I went through over the weekend with mine, the SD card is where I'd start looking. It could be the power supply too but if you're seeing the same thing with 2 different supplies it wouldn't be my first bet.
Also, if you do get it running long enough to get to a command prompt, dmesg may have something on what's going on.
It's a nice little board once you get past the teething problems, don't give up!