The other day, I stumbled on a nifty way to import and export xlsx files into and out of RapidMiner.
There's an R package called "xlsx" that is able to read and write Excel files including Excel 2007 xlsx format.
Here's a process that loads the iris data, writes this to an xlsx file at c:\temp\iris.xlsx and then reads it back again.
(Note: after the first run, comment out the line in the R script that installs the xlsx package to avoid downloading the R package again)
Edit: 12/7/12: The new version of RapidMiner supports this - hurrah!