validates_uniqueness_of in 2.1

I updated an application from Activerecord 2.0.2.9216 to 2.1.0 and now
validates_uniqueness_of doesn't seem to be working. The unique
constraint was caught by the activerecord validation in 2.0.2, but now
it's getting past that and being caught by the database instead.

When I do a gem 'activerecord' '=2.0.2.9216' before activerecord
require it works again.