The thing I find odd about Ryan as a choice is that, up until now, Romney has offered so little information on what sort of budget he'd create. He has mostly been either nonspecific or just given very broad details. I sort of wondered if that was a tactic so he could basically just run on the "Hey, at least I'm not Obama!" thing. Since "generic Republican" has been the only candidate that has been able to defeat Obama in the polls, I figured he was trying to be as generic as possible by just hitting on the various bulletpoints that appeal to the GOP without getting into too many specifics.
By picking Ryan as his running mate, however, he is essentially allowing the Ryan budget to become the defacto budget of the Romney campaign. It will certainly give the Obama campaign a lot more specifics to attack him on, which has the potential to put Romney on the defensive almost immediately. I don't see how he has Ryan as his VP pick without either having to defend the Ryan budget or coming up with a complete budget of his own.