DHH's acts_as_taggable plugin is basically deprecated -- in fact it was largely cooked up as a demo:
The plugin was intended as a demonstration of has_many :through, I'm not sure that adding more features gets us any closer to that goal. As with other domain-intermingling plugins, it's unlikely to be able to solve most people's problems without serious hackery. Perhaps it's best left as is, but with the documentation updated to make it clear what the intention was.
(Koz from Rails Core)
Despite that fact tons of people are using it in production.
Evan Weaver has cooked up a nice alternative, and an article describing how to use it.