It's an interesting option. As readers of our ECM Suites Report 2008 know, Alfresco is one of the more interesting ECM vendors around. The question you might ask yourself however is, why would anyone want to do use Alfresco instead of SharePoint? Clearly open source enthusiasts will herald this as a major breakthrough - and those enterprises that espouse open source may well become customers. But then again, if you are going to dump Microsoft for the back end, why use Office at all, given there are open source alternatives?
But this release has appeal beyond the open source community, for the simple fact that by using the Alfresco alternative you are not locked into the Microsoft stack. This isn't so much an issue now, but will be when Office and SharePoint effectively merge at the next major release. Unhooking the two from each other (or at least having the option to do so) is good risk mitigation, and a powerful thing to have in your armory when negotiating with Microsoft.
Just as clearly Microsoft would like to lock you in with their solution.