I enjoy the cynical thinking that this marriage is so Ms. Sarah Palin may run for President in 2012 with no outstanding family issues.
However, after thinking about it, I doubt Ms. Palin had anything to do with it for these reasons: 1) the enmity between Ms. Palin and Levi is well known. I believe that Levi has even sold Palin secrets in return for cash (I know he threatened to at one point). If Ms. Palin went to Levi and promised him rewards for his marrying Bristol, that simply gives him blackmail power. Indeed, Ms. Palin would forever be in his power.
2) it does seem that Bristol and Levi kept it secret from Bristols parents, at least until announcing same on nationwide television. That is inconsistent behavior with the thought that Ms. Palin is behind the marriage. You two get married so I wont look like an idiot on the campaign trail. Oh! And go on nationwide television and say that you havent told me yet because you are scared of me!
Here is my cynical opinion: Bristol and Levi came up with the idea to get married so they can pitch a reality show (following them as they prepare for marriage, etc). They want money.