Got an interesting link in an email newsletter as in the subject above- well worth a read for those who do not know about it - http://www.sitepronews.com/archives/2007/aug/24prt.html
Yes, this is a real threat nowadays. Some simple steps to reduce the chance are - make all your links absolute rather than relative, and add the META tag BASE HREF with the HEAD part of your Page.
Just finished the read and I had the same thinking as you. Especially since in my line of internet marketing, my competition is huge and making myself stick out above all those other seemingly generic sites out there is hard enough.. Possible attack is, well, very possible. I will try and implement as soon as I can but not in much rush as my site from what I've seen isn't being copped. I'm not sure of the entire process as far as the php coding will go and what not.. So, if anyone looks into this more or already has, shed some light! Good luck.
Here's one real time example of a proxy page that took over other people web page. Use the site: command to see for yourself how 'hi-tech' Google Bot really is. s195474454.onlinehome DOT us -Had your website been one of those victims, lately?
I've outlined another approach to this problem here: http://forums.digitalpoint.com/showthread.php?p=4698107