Seems not to be a module but a code in the index.php and maybe also some others too. When you mean the pop up JS thing, this is seeming to be a bunch of codes you have to port. Do not know what exactly but you can achieve similar things with JA Highslide like used in JA Teline II and some other templates too. There you can also define how the pop up will look like, white black, gold, glowing edges, plain edges and especially what type of login you want to use inside the Highslide like the normal login, a VM Login module or CB Login and so on and so on…
Now on http://www.joomlacache.com you can download a free Highslide CB login like this. Saves you coding work and in general the Highslide version gives you more possibilities in how you want to style pop up etc…
Much success with it anyway!
P.S.: When they have changed this to a real module, then good, the last time I looked it was not a module but a bunch of codes, and this was like last week or so..