It's true, and it's particularly true for the prefrontal cortex since the dopamine transporter is mainly in regions of the middle of the brain but not the frontal cortex, meaning that catecholamine O methyltransferase plays a larger role in the frontal cortex:
http://www.anthrogenica.com/showthread.php?8-COMT-variation-and-intelligence-(rs4680)
The difference between Val/val and Met/met alleles will result in a 3-4 fold difference in dopamine catabolism. I myself am Met/met, and since joining this forum have been highly cautious of using any methylation agents. I have had severe panic attacks in reaction to caffeine which I have avoided ever since. There are many many many genetic factors that can lead to increased dopamine here or there (various regions of the brain, as this stuff isn't as 2 dimensional as forums sometimes make it seem, or wish it to be) but the rs4680 I believe is a particularly powerful SNP in its ability to alter dopamine concentrations. Here is a pubmed article discussing the 3-4 fold difference between val/met homozygotes for rs4680 in the summary (just for evidence).
http://www.ncbi.nlm.nih.gov/pubmed/16395295
