This exact question was posted on SERoundtable and Google says the answer is an overwhelming yes. Several Google representatives all referenced a study that Google did last year that showed that sites who experienced high organic rankings and also participated in Adwords experienced 20% more overall traffic than doing one or the other. Google also says that CTR on Adwords ads is higher.
Of course you have to consider the source but I have also experienced the same results with my clients. Since the organic rankings have been in good standing, the CTRs and traffic has increased as well. In almost any case, if you rank organically, I WOULD also bid on those keywords with PPC. It has a great branding effect on users who are able to identify the URL all over the page. Simple put, in the users’ eyes it qualifies that sites existence on the page. In very rare cases where the keywords are extremely expensive and with a low conversion rate, it may not be beneficial.