We compare the metallicities of stars with radial velocity planets to the metallicity of a sample of field dwarfs. We confirm recent work indicating that the stars-with-planets sample as a whole is iron-rich. However, the lowest mass stars tend to be iron-poor, with several having [Fe/H] < -0.2, demonstrating that high metallicity is not required for the formation of short-period Jupiter-mass planets. We show that the average [Fe/H] increases with increasing stellar mass (for masses below 1.2 M) in both samples, but that the increase is much more rapid in the stars-with-planets sample. The variation of metallicity with stellar age also differs between the two samples. We examine possible selection effects related to variations in the sensitivity of radial velocity surveys with stellar mass, apparent magnitude, and stellar metallicity, and identify a color cutoff (B-V 0.48) that contributes to but does not explain the mass-metallicity trend in the stars-with-planets sample. We use Monte Carlo models to show that adding an average of ~5 M_⊕ of iron to each star can explain both the mass-metallicity and the age-metallicity relations of the stars-with-planets sample. However, for at least one star, HD 38529, there is good evidence that the bulk metallicity is high. We conclude that the observed metallicities and metallicity trends are likely the result of the interaction of three effects: accretion of ~5 M_⊕ of iron-rich material, selection effects, and high intrinsic metallicity.