We use luntbuild. It's a web app that's very easy to use. It will check out from CVS/SVN, automatically increment the version/build number, execute build tasks in ant scripts and tag your repository with the new version. Can schedule automatic builds, have it email you or contact you via IM, also has security and build dependencies.
We're still using version 1.2.3 but I see it's up to 1.6.0. It just works.
http://luntbuild.javaforge.com/
EDIT: Re-reading your question I see now that you're looking for something to replace Ant. In our case Ant is not really a problem. We have projects setup in Netbeans, which uses Ant for building and we just have to implement some hooks into the existing scripts provided by Netbeans which is very easy to do.
EDIT: Looks like you can call Ant from Groovy. That would be nice because then you can re-use all the tasks that already exist for Ant.
http://groovy.codehaus.org/Using+Ant+from+Groovy
