I just received a BBB and had the same problem as described above. It is now September as I write. My BBB system Reference manual Rev A5.2, on page 18 in 3.4.2 describing setting up the board in stand-alone configuration, directs in big red letters not to plug the uHDMI to the board until the board has fully booted. Geralds says the board senses the video mode from the TV. This obviously can’t happen if disconnected. After the board was booted and the HDMI video was indeed not working (TV reports no signal), I hit the reset button on the BBB and almost immediately got the BBB logo on screen. The board continued and booted to the default screen as shown in figure 16. So, is the manual out of date on this point?
Follow the debugging guide as written on the support Wiki. http://circuitco.com/support/index.php?title=BeagleBoneBlack_HDMI and you will see that it says to do exactly what you did.
The issue is this. Some monitor are not valid and they source voltage on the 5V pins where they should not. This is just a precaution to guard against that and to take it a step at a time.
Fell free to plug the board in before power up if you see no issues.