In all likelihood, they had prepared similar ads for all of Obama's prospective selections. Once the announcement was made, it was just a matter of rolling out the right one.
That's even better. Demonstrates that McCain's peaople are planning for a number of contingencies. I believe McCain is going to keep Obama on his heels the rest of the way, forcing Obama to react to McCain's message and ads rather than being proactive.
The more Obama has to respond, the more McCain's message gets out, thereby multiplying the investment in McCain's ads. The media will have to play McCain's ads for free in order to put Obama's reaction into context.