This issue is identical to the one discussed a while back in Lighthouse http://datamapper.lighthouseapp.com/projects/20609-datamapper/tickets/470-memcaching-can-t-marshal-dm-objects#ticket-470-21
See a test that illustrates the problem here: https://gist.github.com/1596396
The problem does not occur if dm-core and friends are explicitly required. Maybe require 'data_mapper' is out of style these days?
The issue is that require 'data_mapper' pulls in dm-validations which is the ultimate culprit.
Here's a pull request for a fix in the 1.2 line datamapper/dm-validations#52