lucy24 - 6:35 am on Mar 19, 2013 (gmt 0)
especially when Bing and NOT coming from a legitimate MS IP offering valid DNS
Or, in bing's case, vice versa: bing/MSN IP but not bing/msnbot UA ;)
Depending on your site setup, you may also find in convenient to block external requests for files in .php. Just make sure you keep any redirect:rewrite pairs.