Excel 2007 uses ZIP compressed XML, labelled Office Open XML. It roughly looks like the open standard ODF used in free software, but it seems Microsoft intentionally made its format incompatible with the ODF format.

As both standards are based on XML, and both are more or less documented, you "simply" generate some XML, ZIP it, and rename it to *.xlsx. (Perhaps you have to sacrifice some chickens, burn some candles, and draw some blood pentagrams on that way. But you should be used to such procedures when you deal with XML and/or Microsoft software, so nothing special here. ;-)) The OpenOffice::OODoc package should at least be able to read and write the ODF format without hurting animals.

Alexander