This issue has come up several times. Here is what USCIS says:

USCIS Considerations to expedite Cases

All expedite requests are reviewed on a case-by-case basis, and are granted at the discretion of the Director. The criteria are as follows:

* Severe financial loss to company or individual
* Extreme emergent situation
* Humanitarian situation
* Nonprofit status of requesting organization in furtherance of the cultural and social interests of the United States

The news item and related documents are available here:
http://www.immigration.com/newslette...prmprocss.html

Please note, for now (June 2008), premium processing is only available if you meet ALL of the current requirements:
1. Your are currently on H-1;
2. Your H-1 status is expiring within 60 days when you apply for I-140 premium processing; and