In this article Steve Zara (director of UK software consultancy company Zac Parkplatz Ltd.) discusses the implications of integrating db4o into Grails.
Steve states that db4o's transparent persistence is a requirement for a Grails integration. While experimenting with different cache systems db4o yielded impressive results (Steve will blog about this soon, stay tuned).
His general overview is that with the latest version of db4o: "things are looking good for the possibility of integrating db4o into Grails, giving a fully object-oriented system throughout the code, and into web frameworks in general".
db4o could become part of Grails' persistence plugins and a serious alternative to GORM (Grails OR mapper).
Way to go Steve! (we fully support this project =)