It appears that tablets are redirecting to the desktop/PC site. Since
we don't have an experience optimized for tablets, give tablet users a
link where they can easily switch to the mobile gateway (other sites
appear to be doing this as well).
Last I checked, tablets are supposed to go to the desktop/PC view because the small-screen view looks and feels horrible on them, whereas the regular desktop interface works very nicely on tablets -- so it sounds like you are seeing the expected behavior.
We ought to have a link for swapping available to _all_ readers, but I don't know if that's in place.