There’s a few other ways to do this. Haven’t seen an issue with window.open (yet), but you could also create a standard href and manipulate the src attribute of that href as you wish. Here’s that technique: Ads: Include at least one ‘ClickTag’ in your html?
And here’s a few more, most of which use the window.open technique: http://forums.tumult.com/search?q=clicktag