I found the problem:
In the mozodojo branch, the default path for the smbios.plist file is /Extra/smbios.plist. However, in the trunk, the default path for the smbios.plist is /Extra/SMBIOS.plist. Since I had /Extra/smbios.plist and not /Extra/SMBIOS.plist, my smbios.plist was being ignored, along with its setting of MacBook5,1 as SMproductname. Instead, it was using the MacBookPro4,1 default in smbios_patcher.c. Renaming /Extra/smbios.plist to /Extra/SMBIOS.plist got SpeedStep working again, now with the trunk build of Chameleon.