Here is a simple test case which creates 5x5 table and adds 2 merged regions which cross each other. The resulting Excel file is displayed as expected by OpenOffice 1.1.1. Using Excel, however, the complete table becomes merged. I've tried with POI 2.0 and 2.5. The version of Excel I'm using is Excel 2002 from Office XP. When the test case is modified so that the regions do not overlap, the result looks fine.
Created attachment 11877 [details] Java test case
Created attachment 11878 [details] Excel file produced by POI 2.5
I've tried to create overlapping regions directly in Excel it is seams it just can't do that (while OpenOffice can). So this isn't a POI bug. Sorry for the bafflement.