Implementation not provided yet (as of 3.1-beta1+ svn r657702). Another minor problem is that POI does not mark up the CHOOSE() encoding with tAttrChoose and tAttrSkip tokens (as outlined in excelfileformat.pdf sec 3.10.5). Excel (2007) seems to be OK with this. The fix for this will involve similar logic as currently applied for IF() in FormulaParser.getFunction(String, int, List) etc. This is probably not worth fixing until a real case is encountered where it is a problem.
Fixed in svn r730469. Testcases updated. Token markup changes has not been implemented yet.