I do have the same problem: I did not use CS4 for a while but today, it was unpossible to export any clip to Encore. The window did not pop-up and the CS4 application crashed each time after the export to media command.
I did a software update with all involved parts: Vista, Premeire pro (4,1,0) and Encore (4.0.1.058) but the results are always the same. Its very frustrating.
One remark more: I do have from time to time an error message for Vista concerning the color setup.
Hope someone can help me.