Just to save anyone the time and hassle, I just lost 1/2 hour of crunching to try the latest AMD 11.3 drivers they just put on the site yesterday. I was hoping it would fix whatever bug in 11.2 doesn't allow my 5870 to crunch DNETC. No dice. The new 11.3 drivers even make it hang on MW.
What's odd, is that the 11.2 drivers worked great on the 4870, but not for crunching on the 5870.

So...it's back to 10.12 drivers for my 5870 rig, and hope this little post here can save someone else the wasted effort.