As for whether or not full Drexlerian nano, including working assemblers, is possible, so far there has never yet been a serious refutation of the work of Dr. Drexler and his coworkers. An attempt to debunk nanotechnology in Scientific American was answered with a powerful reply, which caused the editors of Scientific American to ultimately back down. Here are the links to the responses to both of the Scientific American articles from Sept. 2001:
Here is the entire history of responses and back and forth between Foresight and Scientific American back in 1996:
This also includes some interesting references to some apparent inconsistencies in Scientific American's positions versus its advertising and blurbs in other articles.
On the Foresight Institute website, one will be able to find a plethora of articles and info on nanotech including the online versions of both Engines of Creation and Unbounding the Future and Nanomedicine Vol 1. It also provides the option of purchasing Nanosystems online if you want the relevant arguments with all the math.
In saying all this, we in no way wish to disrespect the otherwise superb and very informative Scientific American, which is read and enjoyed by a number of us here.