I have made a small patch to be able to export results into excel files.
It does not use the apache POI, but the JExcelApi from Andy Kahn. (Licensed under LGPL)
Currently this patch does almost the same thing as the CSV export (I did not even cleanup the export dialog)
It also export anything as string values, so numeric values, booleans, dates etc. are exported as string in excel.
Probably this could be refactored to be a plugin, but actually I had no time to look at the plugin architecture.