Of all the boring white guys Mitt Romney could pick for a vice president, Paul Ryan is the most daring boring white guy he could choose. He's young—23 years younger than Romney—and he's the one with the most policies on the record. The Ryan budget basically ends Medicare (while ensuring that all the older voting conservatives who rely on Medicare will get to stay on it for the rest of their lives) and cuts safety net programs like food stamps and other Federal assistance programs.
So assuming the Huffington Post is right and it is Ryan, this is Romney's last attempt to convince Republicans he's the real deal. Ryan is beloved by teabaggers, but he's not especially well-known for being tough on social issues, which is helpful for winning independents who don't favor government intervention on issues like gay marriage and abortion.
A Ryan choice for VP means that Romney wants this election to be all about the economy, and he thinks most Americans want much less government in their lives. They're going to try to keep the deficit as the most important issue in the campaign, and they're going to try to convince people that the only way to handle the deficit is by cutting back on government. The Obama campaign won't have to do much repositioning of their message to make up for a Ryan choice—they've been preparing for this kind of a race the whole time. We'll find out if this Ryan gossip is true in the morning; there'll be a lot to discuss if it is.