“When connected to USB only I get +4.6v out of the +5v header pin on the Domino (just as brett reports). Interestingly an Arduino Duemilanove connected to the same computer (USB only) gives +5v on the same header pin.”
That is normal, FEZ Domino is a 72Mhz 32bit ARM processor and so it draws a lot more power so you will see a bit less than 5V
“So its ok to connect power to the “power input barrel connector” at the same time that I am connected to USB?”
Yes it is okay.
"The particular shield that I am playing with uses 3.3v but does have one pull-up resistor connected to +5v. So if it is ok to connect power + USB (at the same time) then I can give that a try to see if that is the cause of my SPI comm issue with the shield."
Yes it is okay, just connect it.
What shield are you using?
“Page 1 of the FEZ Domino brochure indicates that the +5v header pin only offers +5v when connected to an external power supply (and not just USB).”
We need to update the manual then.
"So what I really want to know is if powering via external power while connected to to USB is kosher."
Yes you are good to go