A bit late in replying, but just in case NewOrtho is subscribed to this thread:
Of course we believe that the prayers of the faithful are effective and so by having the Icon on the altar for 40 days the Icon is cleansed and blessed for veneration.
... a more mundane reason for this practice is that when a new Icon is painted, giving it to a priest to be blessed means a member of the clergy gets to have a good look at it and check that it is canonical. This all depends on the priest recognizing what is an "Orthodox" icon and what isn't, so it isn't infallible, but it is an extra check put in place to weed out unorthodox images from Church use.
I think this is the overriding reason for the practice, rather than any belief that an icon is not good for veneration until it is blessed.