Why is a ketone more nucleophilic than an ester?
The ester carbonyl carbon is a stronger nucleophile and less prone to nucleophilic attack than the carbonyl carbon in a ketone. I think you are trying to understand why the carbonyl in a ketone typically reacts faster with a nucleophile than the carbonyl in an ester.  Look at the resonance structures drawn below.  Both the ketone and ester have a resonance structure that places positive charge on the carbonyl carbon, however you can also draw a third resonance structure for the ester that removes some of the positive charge from the carbonyl carbon and places it on the ester oxygen. This makes the ester carbonyl carbon less positive (more negative, more nucleophilic, less prone to attack by a nucleophile) than the ketone carbonyl carbon (more positive, less nucleophilic, more prone to attack by a nucleophile).  A nucleophile has electrons it would like to share and will react preferentially with centers of low electron density (more positively charged).  Therefor a nucleophile will react faster with a ketone carbonyl than an ester carbonyl.



EDIT

I like Martin's posts, they make me think - and that is always a good thing. Here is where my thinking has taken me:


I believe Martin's approach (basically frontier MO) is fundamentally correct.  I would be more convinced that his actual numbers are meaningful if he were to calculate 1) the carbonyl carbon LUMO coefficient and 2) the HOMO-LUMO separation for the following series of compounds in order to see if his calculated values correlated with this order of reactivity of carbonyl compounds towards nucleophilic attack (we could use water as the nucleophile) - but Martin, I think such calculations might be a poor use of your time and resources.


acyl halide > acid anhydride > aldehyde > ketone > ester ~ carboxylic acid > amide > carboxylate ion


My argument based on how resonance affects the charge density at the carbonyl carbon is one approach used to explain ("teach") the above order of reactivity.  See for example page 4 on these UCLA notes.  I was just trying to keep it simple.
The issue I have with Martin's approach is, what should the reader do next week when he wants to know if an acid chloride reacts faster or slower than an ester.  While the resonance approach may not be rigorous, it does yield the correct order of reactivity and allows the user to make predictions while only using pencil and paper.
I am considering further editing of this post to use the resonance approach to explain differences in ground state stabilization of the carbonyl series.  Then, since they all go (more or less) to the same tetrahedral intermediate, one could explain the reactivity order based on differences in Eact.  This is another textbook method used to explain the carbonyl reactivity pattern.
Finally, what is the "correct" answer?  Is it the real truth as nature knows it, but in a form that is difficult for the common individual to use and extend; or is it a rough approximation of the truth that can be easily used?  I think the answer to that question depends upon who is asking the question, who is the audience.