"Invalid partition table" error when creating more than one slice during install?

[Darren Pilgrim]

I'm trying to get 5.1-R installed, but I've run into a problem I don't
understand.  The disk layout I want should look like this:

s1: 20gb, FreeBSD
s2: 10gb, Windows
s3: ~83gb (the rest of the disk), data

The partition, label, and install process all seems to work fine.  However, when
I reboot, rather than beginning the bootstrap, the machine halts with the error
"Invalid partition table".  If I install with just s1, it works fine.