Addendum:  Some pundit comments.
Eszra Klein:

1. Both Democrats and conservatives are going to get the exact debate they wanted. I’m not so sure about Republicans.

2. This is an admission of fear from the Romney campaign. You don’t make a risky pick like Paul Ryan if you think the fundamentals favor your candidate. You make a risky pick like Paul Ryan if you think the fundamentals don’t favor your candidate. And, right now, the numbers don’t look good for Romney: Obama leads in the Real Clear Politics average of polls by more than four percentage points — his largest lead since April.

Nate Silver:

When a prudent candidate like Mitt Romney picks someone like Representative Paul D. Ryan of Wisconsin as his running mate, it suggests that he felt he held a losing position against President Obama. The theme that Mr. Romney’s campaign has emphasized for months and months — that the president has failed as an economic leader — may have persuaded 47 or 48 or 49 percent of voters to back him, he seems to have concluded. But not 50.1 percent of them, and not enough for Mr. Romney to secure 270 electoral votes.

US News:

There’s an argument that the three polls out yesterday giving Obama an outside-the-margin-of-error lead could also spur a game-changing pick a la Ryan. “The conventional wisdom had been that Romney was going to be picking a running mate in a coin-flip race. Well that’s not the case now. How does that change his mind? Does it help Paul Ryan?” asks “First Read,” adding that Romney has gone from picking a running mate from a position of strength to “picking one from a position of weakness.” That seems a bit strong, especially based on one set of polls. Does the Romney team want to exacerbate a perception of weakness by making what could be seen as a panicky pick (a sop to a jittery base, a Hail Mary in the face of a widening gap in the polls, and a whiplash-inducing strategic change from deliberate policy vagueness to a highly controversial off-the-shelf economic agenda).

Seems to be a consensus.

On the upside for Romney, it may bring Wisconsin back into play.