Hi Gavin,
The short answer is "no". When the popup is created it's just a stub, so has no content, so doesn't know how high it will need to be when it opens.
So it opens centred (you specify the width) and "top aligned".
That said, it is possible to "push" it down, by setting the option for the jQuery Dialog (check the jQueryUI docs for more in that direction.) Be careful though, allow for screens of _very_ different sizes.
Cheers
Bruce