Yay! I finally got this to work, but the instructions in HOWTO41864 appear to be slightly off. The article says to follow KB article TECH139702 exactly beyond changes made to Requirement 1 in that document. However, Requirement 2 says you MUST assign the client an IP address - ANY IP address valid in the subnet, and the client will update the next time it boots. This is incorrect. I was unable to get my clients to import correctly if I included DHCP = 1 and specified an IP address.
So, what is correct is that if you are importing a Predefined Computer using DHCP, in your spreadsheet you must have DHCP enabled and no ip address entered. Again, not a big issue but it is behavior slightly different than what is described in Requirement 2 in TECH139702.
EDIT: Before I speak ill of Thomas' instructions, I just realized I had a second Task Server on the same subnet that may have been causing me conflicts when importing computers. So technically speaking, I don't know for a fact that entering DHCP enabled along with an IP address caused my problems importing. I do know for a fact, however, that it does work to enter DHCP enabled without also entering an IP address and subnet.