Challenge? Not really.
I read the Challenging Microsoft Office piece with the usual curiosity I have about what people think about MS Office alternatives. Anyway, the thing that jumped out at me about this story was a huge weakness/drawback that the author hinted at but didn’t explicitly mention: lack of integration. Other than OpenOffice, all of the applications considered are task specific applications, not really Office suites. Try doing a mail merge in Writely with contacts stored in Thunderbird. Not gonna’ happen. The applications mentioned may be useful to consumers with occasional needs, but for the typical corporate user it's not in the cards.