If there's a prize for being the first to point out the following, I 
hereby claim it:

Smith's actual words are "[the merchant] is in this, as in many other 
cases, led by an invisible hand [etc.]" -- no "as if" about it (WN 
IV.ii.9: page 456).

In consolation, this common mistake has been committed by no less 
figure than Joseph Stiglitz; see:

http://www.newstatesman.com/business/2008/10/economy-world-crisis-financial

Further, as Gavin Kennedy points out in a comment on this page (16 
October 2008 at 11:58), Smith uses the metaphor -- in the only place 
in which he does use it -- in relation to the allocation of capital 
guided by the merchant's pursuit of profit, not in relation to supply 
and demand or any other manifestation of market processes.

On a constructive note, there would be an interesting project in 
trying to find out whether this misquotation had any currency before 
Milton Friedman published his notorious methodological essay.

Julian Wells