I want to start targetting some long tail search terms - now if I target a keyword like 'super duper yellow balloons' (as a ridiculous example), given that the search popularity for such a term is likely to be very very low - is it sufficient to just have 1 link containing this anchor text to be seen as relevant to this search -- or is it the case that you would need a few links to be seen by G as an authority (lol) site on super duper yellow balloons? Now obviously the type of link will be a factor - but would a couple of links from 0 PR directories be sufficient?
Deep links are not the best way to target the long tail. The whole idea of long tail is loads (hundreds or thousands) of pages that get 1 or 2 hits per day. You can't build anchor text links for this many pages its impossible. You need to build your domain into an authority site using quality content and links and then make sure the pages on the site are targetted to the long tail search terms.