I'd think they would only really have an effect on the higher end market. Ie people with similar amounts of wealth see what the celebrity buys, and it hope of achieving the same status follows suit. I guess that would probably trickle down to collectors like us, increasing prices of prints and whatnot.

I will say that more often than not tastes seem to be influenced by popularity rather than the strength of the artwork. The case in point is Mr. Brainwash. I recently saw him appear with both the Kardashians and the Obamas. Its easy to see why the "urban" section of most contemporary galleries contain mostly his artwork, while most serious collectors won't go near it.

Anyone who allows themselves to be the subject of a Comedy Central roast can't be all that bad, or that intelligent come to think of it. 

I would expect that David Cameron hadn't even heard of Eine when the gist was given to Obama. I could be wrong though.

It all depends on the artist and how they exploit the celebrity bought their art. Banksy for example uses a PR team to tell the world. Other artists think it opens doors by assosciation. Plus it can mean that galleries want t sell and promte the artists work. Thats the shallow end of the art world where artists churn out stuff and become a brand where money becomes more important than art. At least one celebrity has my art and it makes no difference to me personally.

On a personal note I couldn't care less about celebrities taste in art they are vacuous by default. I don't have idols in that way. In short I think I have my own tastes. If people want to follow a celeb, politician then that's fine but I'll make my own mind up!