Given that entry line and enthusiast AMD gfx cards scale in their mhash per Watt, I see zero argument / sense in differentiating between "hobby" miners vs "pros".
How could there be? The MARGIN stays the same issue regardless of how much VOLUME you have.
If it somehow becomes unprofitable for the current AMD generation of gfx cards, it becomes unprofitable for _ALL_ of them.
Regardless of whether it is some dude running 10 machines of 6990s or one normal guy with a 5770 - setting aside other hardware components / power draw sources, of course.
Yeah, I was thinking the same thing as well, just neglected to put it in writing.
It really depends on MHash/watts as far as whether a given setup will remain profitable. But if you have a rig with 4 of the most efficient cards on the planet, then you could stay in business just as easily as a "professional" miner would.