Alfresco, a start-up founded by John Newton (co-founder of Documentum) just got $2MM in seed round financing from Accel Partners. They want to become the MySQL of Enterprise Content Management and promise an initial release of the product for download on July 27th 2005. The web site is a little thin but some information can be found on the road map page and the architecture page. They have made a great selection of OSS components: Lucene search, Spring AOP framework, and JCR (Java Content Repository) support and the application will run on JBOSS.
It is unclear if/how an external community will contribute to the project - that area of the site is not yet live. However, none of the people listed on the team page have a history of open source project leadership so it is possible that development might be limited to Alfresco engineering staff.
I am looking forward to trying Alfresco. There are few major open source CMS projects focusing on document management despite the fact that document management represents a major part of the content management market.