I cache partial templates in my app by registering a fragment store:
register(:fragment_store, Merb::Cache::FileStore, :dir =>
Merb.root / :tmp / :fragments)
and using it in my templates (haml) like:
-fetch_fragment({:cache_key => "some-path/#
{@some_item.item_identifier}"}) do
=partial 'some_template'
You may have already found this, but there are some docs at:
http://wiki.merbivore.com/cache
Take a look at the example apps at github.
Vince