Matthew Turland has posted about a topic that was the subject of a recent cover story in php|architect magazine - EAV modeling.

The mental image that came to me when reading about this approach to data modeling was taking the traditional relational table and turning it on its head. [...] EAV actually has to circumvent, work around, or reimplement features that most mainstream database servers today provide "for free" to the traditional relational counterparts of EAV in order to get equal functionality.

He talks about features the structure has (including row-level referential integrity and schema metadata) and points out some of the other document-focused database systems like Lucene and CouchDB.

Check out the story in the June 2008 issue of php|architect for that cover story from Carl Welch.