I remember having a similar problem when Leopard came out… trying to print a label from usps.com would look like it’s working but the print dialog box would never appear.
There soon came a workaround, clicking the bottom left corner of the screen (a hidden button, basically) would allow the label to be opened in Preview.
The solution is to use the web browser Opera. I’ve never needed or used Opera before because Safari or Firefox worked for every website. But now with 10.6, I’ll be keeping Opera around in case a situation like this happens again.
You can download the latest version of Opera 10 here.