DMZ 1.7.1 (DataMapper OverZealous Edition)

[eluser]WanWizard[/eluser]
Check the server requirements: minumum PHP version for Datamapper is 5.1.2.
Someone managed to get it to work with 5.0.0 (see http://ellislab.com/forums/viewreply/728767/), but no guarantees.

I understand what you mean now with set_value(). Would you please report the issue at http://bitbucket.org/wanwizard/datamapper/issues, so it doesn't get lost? Add a link to this thread.
I'll see if I can come up with something.