Looks like Alfresco is starting to pursue JSR-170 for its 1.2 release. The positive side is it clusters, the negative side is that at the moment it doesn’t pass all the Level 2 170 tests. I could be plugged in under the new CHS service with a small adapter, but, if its not solid through the JSR-170 API, then it will be a pain to run, even in a cluster.
One to watch and put under the Content Hosting Service when its ready, if Jackrabbit doesn’t get there first.
T F D 
This information is a bit out of date. Alfresco released 1.3 back in June and it was fully JSR-170 Level 2 compliant. The 1.4 Preview Release is available now.
I agree that Alfresco integration with Sakai as a CHS module is very appealing. More so than Fedora or DSpace integration in my opinion since it would be done using the JSR-170 standard instead of a proprietary API.