Is nagios possibly looking for an HTTP/1.1 request? You may have to add a few things to your send string, along the lines of the following:
GET /nagios/cgi-bin/extinfo.cgi?type=2&host=SERVER1&service=CPU HTTP/1.1\r\nHost: \r\nConnection: Close\r\n
You also might try the SuperHTTP monitor script:
http://devcentral.f5.com/Wiki/default.aspx/AdvDesignConfig/SuperHTTPMonitor.html
It's super flexible and fairly straightforward - the script comments walk though the various options and how to set them.
Ah, also - just remembered. Are you on V10 yet? You might take a look at the inband passive monitor feature that was recently added:
http://devcentral.f5.com/weblogs/amurphy/archive/2009/04/09/5933.aspx
(edited for readability)