The selection of Sarah Palin makes a McCain Presidency slightly more appealing to many conservatives, frustrated with his commitment to open borders and Neo-Conservative-brand Zionism. Many conservatives may cross over to McCain's ticket (from the feminist-friendly Obama-Biden one) . McCain's natural and logical choice would be Joe Lieberman, as he is a fellow Zionist-in-arms; he predictably went against such a choice.
The question is, does Palin change the equation enough to prevent one of the most disasterous decisions McCain will make in office: invading Iran? The election of John McCain basically gives authorisation for an military offensive against Iran, which is a long-term project of his.
My guess is that regardless of whether Palin is strongly opposed or not (probably, she is quite neutral: she has one son in the Iraqi conquest), McCain will have his new war if elected. That's the wider picture and why even Palin's relatively strong conservative credentials is not enough to make the Republican ticket acceptable.