Today I went to the Solutions Linux Open source conference and met the founder of JasperReports: Teodor Danciu. He is Romanian and speaks a very good French.
I had a quick chat with him. First, I thanked him for creating that great reporting library and then I asked him if the generated Excel files are compatible with Excel 2000, being a requirement of the customer on the project i am currently working on. The answer is: check the excel libraries that are used in JasperReports (POI and iText). They are compatible with Excel 2003 for sure.
I quickly discussed with him the support of iReport/JasperReports for EJB-QL queries. And MDX queries against an SQL Server 2008 Analysis Services. It seems a fix is available in the SVN repository and will be integrated in the next release (in 2 weeks) of JasperReports (3.7.x ?)
In the end, he gave me his business card to contact him if needed.