Yes, I know exactly what a master boot record is, jmac.
Typically, grub installs stage 1 of the boot loading mechanism in the MBR. Grub misconfiguration or misuse is the easiest way to cause these symptoms, at least in my experience.
To fix it requires no reinstallation - just a livecd - if this is the case.
Now, if grub is installed differently in Ubuntu than the default (entirely in one primary partition), then it should just be marked active (bootable). If your partition table is totally messed, try the testdisk tool I recommended above.
Btw, what distro's do you all run?
I run gentoo stage-1 installs on all my machines here (laptop + amd64 + older x86), although I'm thinking of running freebsd on the x86 when I get a new hard drive for it.