Recent significant Russian arms sales (including 250 advanced Su-30 fighters) to Iran does put a bit of a different spin on our arms sales to Saudi Arabia. At least, it does so in a purely contemporary pragmatic sense.
Really, what it says is that we should hurry up and build our ground and air bases in Iraq and Afghanistan and start stocking them up. And the $20 billion should have gone straigh to Israel. Thinking that we can trust Saudi Arabia (or any other Arab state, for that matter), with protecting our interests in the Middle East is the kind of thinking that go us where we are today.
We need to stop with the subtle geopolitical wrangling and start acting like the superpower we are. If that means being really, really nice to our allies and really, really mean to our enemies, than so be it. First, of course, we have to identify which is which, and so far, we've done a poor job of it.
Iran's going to be our problem. Saudi Arabia won't do anything for us. And unfortunately, the Russians and Iranians both know this better than do we.