[jQuery] CSS top seems ignored by blockUI

Hi,
I'm passing to blockUI an array of options:
{top: '150px', width: '40%', padding: '2px', border: 'none'};
And top appears to be completed ignored! The other options are used,
but the message is consistently drawn centered vertically; no matter
what I pass as the value for top.
Any thoughts?
thanks,
jab