I am pretty sure there may be a simple php script for this.

Whether it's seen as obtrusive will vary with your market, I think. Just make sure that there's a clear way to close the window without taking the survey.

Yes, there are plenty of exit survey scripts out there, in several scriptiing technologies.

i figured i could search one out there... i was more concerned about the user response...

they seem to not bug me too much if there is that "x" exit option in the box....

I'm relating this to the Microsoft request to tell them if their Help Files in various applications were of use. I rarely give them that feedback, but I don't feel bothered either.

Maybe this is not something you would want to serve to regular users on every visit - you could control that with a cookie so as not to alienate your key visitors.

As mentioned above, depends on your target market.
If you have a specific service site, I think users will not be surprised and many will actually fill out this small survey (keep it small).

The way to get bigger feedback percentage is to make it slick.
I recommend using specialized jquery method,
essential files to google:
jquery-impromptu.1.8.js
jquery.corner.js

It's easy to modify in css and looks pretty.
Good luck : )

Surveys which just pop up really annoy me, and I don't know why!

They seem to be all the rage on e-commerce sites at the moment, I always click the 'no thanks' button.