Rev A vs Rev B first time boot

I just received a Rev B beagle and I am wondering if there is a
difference in boot. I've seen instructions that indicate you must use
the MMC. I read on the wiki that you can boot from the NAND if you've
got a Rev B. Is there already a linux distribution on the Rev B

Should I do the diagnostics test first? It looks like performing this
could result in a reflash of the NAND...

I've tried reading all the posts and documentation, but I'm new to
linux and some of reading went right over my head.

Thanks in advance for the help!

Only an Xloader and UBoot are flashed onto the Rev B boards. There is no distribution of Linux loaded on the boards.

I suggest you start here to get you up and going.