The word on the street at the moment is that it will be the unveiling of the long awaited iTablet or whatever they call it.
The following is posted on CNET's Apple Blog...
Latest Apple Computer News - CNET News
Could it be the new iPhone? 50/50....A "Bigger Screen" could mean the tablet or maybe a higher resolution of screen maybe??? The later would lead me to get my hopes up of iPhone OS 4.0, I wont be getting the 4G anyways until upgrade time....only had the 3GS for about 4 months