The HD 5870 and HD 5870 X2 are not designed for those markets at all. AMD has other products that already/will address those needs.
Now when are those companies going to realize that we need smaller form factor cards? Those are almost too big to fit in any case!
HD 5850 is faster than the GTX 285 (10.5") and measures only 9.5"
HD 5700 series which is as fast as or faster than the previous HD 4800s/GTX 260 measures only 8.5", shorter than the HD 4800 series (9"-9.5") and a hell of a lot shorter than the 260 (10.5").