You need to specify that Darwin boot from the UFS partition, either manually by typing

rd=disk0s?

at the boot prompt, or by editing the file /Library/Preferences/SystemConfiguration/com.apple.Boot.plist and changing the string following "Kernel Flags" from

<string></string>

to

<string>rd=disk0s?</string>

Darwin's booter is rather primitive. Consider using another, more mature bootloader (for example lilo or grub) if you want to boot multiple operating systems, or if you want Darwin to reside on a secondary hard drive.