I think it would be very difficult to put more responsibility on the shoulders of DPS unless they, for example, changed how threat worked such that it would be DPS' job to maintain low threat rather than a tank's job to maintain high threat, but it would be a) difficult to implement and b) probably cause more problems than it would solve since there would be a bunch of people who wouldn't know about the change and would consequently just pull aggro all the time and get mad at tanks about it (which I'm pretty sure is the #1 reason why tanks stop tanking for PUG groups - idiots get mad at them all the time for stupid shit).

Either way, I'm not sure that this is such a bad idea - it will definitely encourage tanks who generally only run in guild groups to throw themselves into the PUG pool once and a while, and they won't be able to bring anyone with them to "take" DPS spots if they want to get the rewards. DPS that can be tanks might certainly attempt to tank, but unlike in Wrath where you could get a very well-geared healer to carry a DPSer in a tanking spec through dungeons, you can't really do that yet in Cataclysm. On the other hand, as gear gets better dungeons should become easier and people will be less "afraid" to tank in PUGs or tank period, so this kind of thing may no longer be needed.