This question is just a sanity check. I couldn't find this information anywhere else.
If I turn on the pi without an SD card attached, does the firmware print any error messages to the screen (via HDMI or RCA) as many PC BIOSes do, or does the red LED serve as the error message?
See http://wiki.beyondlogic.org/index.php?title=Understanding_RaspberryPi_Boot_Process
The GPU is responsible for booting the system. It loads the first stage bootloader from a ROM. The first stage bootloader loads the second stage bootloader (bootcode.bin) from the SD Card.
The second stage bootloader (bootcode.bin) executes on the GPU and loads the third stage bootloader (start.elf)
The third stage bootloader (start.elf) reads config.txt, cmdline.txt, and then loads kernel.img to CPU memory. The CPU then boots from kernel.img.
Without an SD card the first stage bootloader will fail. As far as I am aware it has no knowledge of any other hardware - so no screen output, no serial link output.
does the red LED serve as the error message?
The red LED indicates there's power.
If the firmware and bootloader are loaded successfully, the green ACT light will flash twice. This happens immediately when you plug it in. Booting the OS kernel then proceeds and ACT may continue to flicker as the OS does stuff (subsequent to the two flashes, ACT indicates activity involving the SD card). At this point the kernel should be spitting messages to console so any errors will be reported there.
If there is an error before the kernel is loaded, the ACT blinks three or more times, meaning, according to this:
No SD card is not exactly an error, though; in that case the ACT light will never flash or glow. If it does not and there is an SD card in, the card reader may be broken -- or maybe you did not quite push it in all the way ;)
