Recently, I have found in a paper by Klammer, et al. a statement that p-values should be uniformly distributed. I believe the authors, but cannot understand why it is so. 

Klammer, A. A., Park, C. Y., and Stafford Noble, W. (2009) Statistical Calibration of the SEQUEST XCorr Function. Journal of Proteome Research. 8(4): 2106–2113.

To clarify a bit.  The p-value is uniformly distributed when the null hypothesis is true and all other assumptions are met.  The reason for this is really the definition of alpha as the probability of a type I error.  We want the probability of rejecting a true null hypothesis to be alpha, we reject when the observed $\text{p-value} < \alpha$, the only way this happens for any value of alpha is when the p-value comes from a uniform distribution.  The whole point of using the correct distribution (normal, t, f, chisq, etc.) is to transform from the test statistic to a uniform p-value.  If the null hypothesis is false then the distribution of the p-value will (hopefully) be more weighted towards 0.

The Pvalue.norm.sim and Pvalue.binom.sim functions in the TeachingDemos package for R will simulate several data sets, compute the p-values and plot them to demonstrate this idea. 

Also see: 


  Murdoch, D, Tsai, Y, and Adcock, J (2008). P-Values are Random
  Variables. The American Statistician, 62, 242-245.


for some more details.

Edit:

Since people are still reading this answer and commenting, I thought that I would address @whuber's comment.

It is true that when using a composite null hypothesis like $\mu_1 \leq \mu_2$ that the p-values will only be uniformly distributed when the 2 means are exactly equal and will not be a uniform if $\mu_1$ is any value that is less than $\mu_2$.  This can easily be seen using the Pvalue.norm.sim function and setting it to do a one sided test and simulating with the simulation and hypothesized means different (but in the direction to make the null true).

As far as statistical theory goes, this does not matter.  Consider if I claimed that I am taller than every member of your family, one way to test this claim would be to compare my height to the height of each member of your family one at a time.  Another option would be to find the member of your family that is the tallest and compare their height with mine.  If I am taller than that one person then I am taller than the rest as well and my claim is true, if I am not taller than that one person then my claim is false.  Testing a composite null can be seen as a similar process, rather than testing all the possible combinations where $\mu_1 \leq \mu_2$ we can test just the equality part because if we can reject that $\mu_1 = \mu_2$ in favour of $\mu_1 > \mu_2$ then we know that we can also reject all the possibilities of $\mu_1 < \mu_2$.   If we look at the distribution of p-values for cases where $\mu_1 < \mu_2$ then the distribution will not be perfectly uniform but will have more values closer to 1 than to 0 meaning that the probability of a type I error will be less than the selected $\alpha$ value making it a conservative test.  The uniform becomes the limiting distribution as $\mu_1$ gets closer to $\mu_2$ (the people who are more current on the stat-theory terms could probably state this better in terms of distributional supremum or something like that).  So by constructing our test assuming the equal part of the null even when the null is composite, then we are designing our test to have a probability of a type I error that is at most $\alpha$ for any conditions where the null is true.
