These questions are probably from when BootX was new. Benjamin Herrenshmidt 
has the latest version available from his web page 
(http://calvaweb.calvacom.fr/bh40/) 

When I boot up my computer (powermac 7200) I get into Boot X (no suprise there).
I noticed it is different than when I get into Boot X via Mac OS (not on start 
up, after everything loads and when in Macos). I have Boot X correctly 
configured but when I click to get into Linux, the screen goes black and 
nothing happens. When I get into Linux via Boot X application (not extention) 
I get into Linux Fine. How come the Boot X extention doesn't work for me but 
the Application does?

  I've experienced the same problem with my PowerMac 7200. I have no idea why this model has this problem. When I attempt to boot LinuxPPC from with bootx during the booting of the machine, the screen goes black and it hangs. When the application is used instead, I manage too get the system up.
  I doubt this problem is monitor dependent. I am running a Sony 14" monitor, which I have NEVER had a problem with before. If anyone knows how to kill this problem please help!

I think the 7200 loads a new video driver during boot 
up of MacOS(ie you see the screen reset). The BootX extension, 
as it is, loads before this and thus Linux doesn't 
boot. But the BootX application works fine as the new 
video driver is loaded. Not knowing much about the MacOS 
extension system, I used Resedit to change the TYPE of 
the BootX from scri to INIT. And that was all it took. 
Now I can run my 7200 headless. Yipee.

Ben's page has moved to http://penguinppc.org/~benh/

Sometimes, BootX loads a kernel and whacks-up the screen on a PowerBook.

This will cause the brightness to go to zero - which turns the screen OFF.
The result is that it looks like the PB has HUNG.

Usually, you can press the Brightness-Up button and see your Linux.