PPC Terrorist Attack Bots: Is the internet advertising system vulnerable to terrorist attack?

Is it possible that Google & Yahoo's PPC programs are susceptible to terrorist attack?

 
Here's a question for those more in the know than I; Is it possible to create a untracable bot network to attack the PPC market as a whole?

In reading about how Evil Bot Networks can be configured to any end, I've had a nagging thought that it might be possible for a motivated entity to decide to attack the entire internet advertising market.

From Wired: Attack of the Bots
PC owners have installed tens of millions of personal firewalls and antivirus programs. But bots are infiltrating even protected computers, and they have quickly become a bigger threat than virulent malware like the famously destructive Melissa, I Love You, and Slammer.

PPC fraud is usually perpetrated in one of two ways. False sites running PPC ads with illegal clicks designed to generate revenue, & and competition clicks designed to waste a competitors advertising budget.

Here's a different scenario: 

An network (terrorist of otherwise) decides that they could damage the internet economy by attacking the PPC models inherent vulnerabilites. The idea is not to gain monitarily or drain a competitors funds, it's to create such a glut of ad clicks that the entire model becomes suspect. Advertisers would no longer trust the system since Google and Yahoo would not be able to discern which clicks were valid and which were not. The underlying motivators (monitary or competitive) would not be there and Google / Yahoo would have no 'motive' by which to measure clicks.

A Bot Network Attack:

• Randomly travels the net in ways that mimic consumer usage patterns.
• 'Clicks' on PPC ads in ways that mimic consumers.
• Does it on an incresingly massive scale.

As far as I'm aware, this is entirely possible.

Google / Yahoo have active programs to determine which clicks may be fraudulent, but those systems are predicated on being able to search by motive or other discernable factors. Bot networks use unsuspecting computers that are infected by viruses, screening them from most search methodologies.