Two days ago I wrote an entry on a forum thread at WebmasterWorld about Google Blocking Rank Checkers. Since then, it was features on WebmasterWorld, primarily because GoogleGuy replied to it with the following comment:
That page originally started because when someone has a virus or a trojaned machine, they often don't realize that their machine may be sending queries to Google without them asking. :)
Recently, we've started to improve the software, and it can also detect lots of things like rank checking and other automatic queries to Google. Of course, we don't show the message to everybody that comes onto our radar, but our new software looks pretty good at detecting programmatic queries. If you're using WebPosition or other software that sends queries to Google from a program, I'm not surprised if you're going over a threshold where your queries are on the radar.
If you do see this message, I would recommend not sending so many queries to Google--the volume or type of queries are unusual enough that it's attracting greater scrutiny from us. If you avoid sending queries to Google from a program, you'll be as right as rain.
Since then, others have been noticing this occur. To see what you would see when Google sends you back this remark, click here or view image. A thread at Search Engine Watch Forums discusses the same issue but this time with a simple search on +"powered by phpbb". Organic-Rankings.com has an other image with a image security code requested by Google here.