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.
I think most of these discussions would benefit from differentiating between profitable and "worthwhile".
To use an extreme example:
Few people would consider mining with a single device for a gain of $1.00 per day to be worthwhile even though it's technically profitable. However, the same margin at 2000x that scale might be equally profitable but widely considered worthwhile.
At that scale you could even hire a couple of employees to handle daily operations and set aside a fund to replace the 4 or 5 devices that would fail each week while still turning a tidy profit.