Google wants to know you're human (updated)
Google throws a Captcha in front of users it think might be bots.
Other people have previously seen this block, though we don't hear about this very often. If you manage to set off Google's filter, it should be a pretty innocuous block. Enter the text, and you should be back up and googling in no time.
That is, unless you are vision-impaired, as this blogger ranted back in 2005. In that case, you need an alternate route around the speed bump. Google has one. Click the wheelchair icon on this page. It will read you a series of numbers, overlaid on a background of jibberish. Presumably, a human will be able to pick out the numbers, but a machine will not. This feature does not appear to be active on the block page Wilson stumbled upon.
Update: Here is Google's official response:
In order to maintain a speedy service for everyone, we attempt to filter out searches that are done in rapid succession by automated computer programs. One of the ways we detect and block these sort of searches is by noting when an unusual number or type of queries originate from the same IP address. Blocks for a proxy IP may happen if there are rogue computers behind the proxy that search on Google automatically.
If you're asked to type in a word written in squiggly text (a "captcha"), doing so will enable you to resume your searches. If you are not given this option, please wait a while before you search again.
Also, please double check that neither you nor those on your network are using automated computer programs that are unauthorized to access Google (including software that automatically submits pages, checks rankings, etc.).
As background, Danny Sullivan wrote about this last September.