The BMC2 uses RMCP by default, but ipmitool wants RMCP+. No problem, SuperMicro provides flashable images that do support RMCP+. Great. Only, you will need to boot the machine into MS DOS and have the image (3.2M, too big for a floppy) on a writable medium (so just having it on a CD doesn't work). Joy.
So... I downloaded an MS-DOS boot disk and dd'ed it to a floppy disk, borrowed from the neighbours. Since the machine wasn't installed yet, fortunatly, I could simply fdisk a partition, reboot, format the partition, reboot and boot into an Ubuntu LiveCD, download the flash stuff and copy it to the FAT partition I just created. Then I could boot the MS DOS disk again, c: and start the supplied Setup.bat to flash the IPMI hardware. Lovely.
Didn't work. I have no idea what went wrong, I got no errors on the screen while flashing the hardware, but after the next reboot, IPMI was untracable. I couldn't access it from the OS or with ipmitool or even with the supplied IPMIView20 Java thingy. Now, I sent some mail to the ipmitool-dev mailinglist, describing my problems and got some useful reactions. None of them helped me out, eventually, though.
So I ran the whole RMCP-to-RMCP+ stuff again. Nothing.
I ran the supplied CD again and reinstalled the IPMI stuff. Again, no response at all.
I ran the RMCP-to-RMCP+ again, and lo and behold! It works! I didn't do anything different this time, very sure of it, because I was keeping track of every little thing I did (since I need to do this for several other machines). But it works.
Now to get OpenIPMI working, but I've found possible help with that. Don't know if it'll work, though. We'll see.
IPMI is cool, though, once it works.