ttrentham sez, "I've been using Tor/Privoxy/Vidalia, FoxyProxy and CustomizeGoogle Firefox extensions for the past several weeks when using Google. This morning, I started getting 403 Forbidden responses from Google when I search."
These proxies are programs that help protect your privacy when using the Internet by disguising the origin of your requests. They're often used by people in oppressive regimes (like China, where Google censors its search results to appease the totalitarian government), or by people whose school, work, hotel or home networks are blocked by censorware.
They're also best practices for anyone worried about NSA wiretaps and PATRIOT ACT fishing-expedition warrants served to investigate your browsing activity.
… but your query looks similar to automated requests from a computer virus or spyware application. To protect our users, we can't process your request right now.
We'll restore your access as quickly as possible, so try again soon. In the meantime, if you suspect that your computer or network has been infected, you might want to run a virus checker or spyware remover to make sure that your systems are free of viruses and other spurious software.
We apologize for the inconvenience, and hope we'll see you again on Google."
Update:: Many have written in to say that Google does this whenever a proxy sends too many requests, in order to prevent scraping by bots. I'm sure this is true, but at this moment, every proxy I've tried is blocked — possibly because some monstrous bot has made use of all of them.
But that doesn't change the essential point: Google is fighting bots by compromising its users' privacy — the countermeasure is a form of punishing the innocent to get at the guilty.