When I started working on Bargainspy I wanted to make it fully XHTML compliant. Pop-up windows, or more the use of target="" is disallowed.

I also read something on www.sitepoint.com about how the 'back' button is the 2nd most used button on a web browser, and if you open a destination in a pop-up window then the back button wont work and it (according to Sitepoint) confuses the user, or some users.

Its been hard to decide. So for now I put that icon next to links which link to external website.