To reproduce this behaviour create an FLA in CS4, make 3 keyframes, draw a filled rectangle (or whatever) on each frame then save it somewhere (ie C:\test.fla)
exportPath = fl.scriptURI.substr(0,fl.scriptURI.lastIndexOf("/"))+"/";
fl.getDocumentDOM().exportPNG(exportPath+"Frame.png", TRUE, FALSE);
The first line of code here just gets the location of the jsfl file and removes the filename to give an export path.
The second line export each frame to the same location as the FLA and JSFL files.
This should run fine in CS4 and you will see the following in your save location:
If you change the first boolean in the export command to FALSE and run the script again you should see the PNG export settings dialog box and when you OK it you will get the same export result.
Now open the FLA and JSFL files in CS5.5 and run the JSFL. With the boolean set to TRUE you should get an error (the 8000px error), however if you set it to FALSE so that you get the settings dialog when you OK it the export will run fine.
Change it back to TRUE and you will get an error again.
I also found that switching the 'current frame only' flag (second Boolean) to TRUE works fine in CS5.5 - it only complains about the bitmap size if I try to export a series of frames, it's fine with just one frame.
This means, of course, that a workaround is to loop through the frames and export each one, which is fine and works OK but it takes 10 seconds compared to the split second that CS4 takes to export the series.
10,000 x 10 extra seconds = 27.77 extra hours of publish time!