After updating to Drupal 7.12, a few views utilizing the taxonomy with depth contextual filter started failing. After digging in a bit, I've tracked the issue to views query/output cache. Disabling views' caching fixes the issue. I then went on to attempt to reproduce the issue in a simplistic form.
With just core 7.12, views and ctools, create a few terms in a hierarchy. The export for my view is attached, however it's pretty simple: just a node field view (title field only) and a taxonomy term ID (with depth) contextual filter which receives its default from the taxonomy term id from url. I created a block display and placed the block.
If you now create a few nodes, you'll notice the view works as expected, however, all it takes to reproduce the issue is to turn on Time-based caching. The view preview continues to work as expected however no results come through to the block.