Snow Leopard, Redmond style

I know, 10.6 has been out for ages (hah!), but only now I found some time to try this. Unfortunately, with no luck:

Mac OS X cannot be installed on Macintosh HD, because this disk
cannot be used to start up your computer.
Huh? I just started from this very "Macintosh HD"! Turns out, others had this problem too and there are a few good and silly articles on the net, but all hints failed so far. Attempting to resize the startup disk's partition via DiskUtility brought up another error message:
MediaKit reports no such partition
Of course, by then I remembered I had rEFIt installed which might confuse DiskUtility et al. - but even after removing it (and a few reboots later), both errors stuck. Although I have a GUID partition table, I tried to update the MBR of my startup disk but I doubt it did anything at all:
$ sudo fdisk -u /dev/rdisk0
fdisk: could not open MBR file /usr/standalone/i386/boot0: No such file or directory
So here we are, the seventh incarnation of OS X but the error messages still suck and are not usable at all :-\