There is certainly tension between Saudi Arabia and Iran, even if the Saudis aren’t tense enough to grant Israel overflight rights, as alleged several times over the past year. Those tensions affect the United States, both in terms of regional security and energy security. Anthony Cordesman, who holds the Burke Chairs in Strategy, Saudi Arabia & the Gulf, and Middle East Energy & Security at the Center for Strategic and International Studies (CSIS) offers his views:

The New Saudi Arms Deal
Serving Vital US Security Interests
Anthony H. Cordesman

One of the most critical strategic decisions the US will have to make over the next few years is how to reshape its security posture in the Gulf and the Middle East as it fully withdraws from Iraq. There is no possible “end state” to the US presence in the Gulf, or an end to the need for the strongest possible US security ties to friendly states in the region.
…

Another take on the issue, from Space Daily:

Saudis amass U.S. weapons to confront Iran