Hi Kevin,
At this stage no, and I suspect the answer is probably gonna always be "no".
The problem is not in the window popping up, that's easy. The problem is two-fold (and these appy to the calendar popup as well, but it's a tad less serious there.)
a) there are no "browser modal windows". It can be faked, but that requires a substantially different architecture underneath. (which yes, may I'll do at some point). So if, for example, the lookup window pops up then the user clicks on the "normal browser window" then the popup goes to the back, _but is still open_. With the date popup this isn't serious, but it could get serious if the approach is used with more important lookups.
b) there's no (easy) mechanism for the 2 windows to communicate with each other. Which makes the return value less certain. Someone saw this effect recently with FF browsers that have specifically been set to "open in new tab, not in popup window".
So in short, if we do support popup-lookups then it'll need some change to the way things work under the hood.
Cheers
Bruce