window.showModalDialog() is not working properly on Google Chrome, this is why you have 3 options here:
1) Put onblur="this.focus()" in the body element of the target page and put a div which will prevent user from clicking on the opener page
2) Use Modal Dialog from jQuery, I have implemented a jQuery function which will give you the possibility to pass arguments to the target page, to close the dialog using window.close() and to return retunrValue
My Article: http://extremedev.blogspot.com/2011/03/windowshowmodaldialog-cross-browser-new.html
3) Do not use Chrome, which I think that is not the best solution.