Question: would this be a breaking, hard change for POMs, or a soft change, (e.g. any of the following specified as an attribute will serve as the declaration unless overridden by a child element, groupId, artifactId, version, classifier, type, scope).
If going down the hard-change route, it would be really, really easy for IDEs (and/or Maven itself ) to support a 1-click update for existing POMs, and would avoid confusing situations with conflicting declarations in attributes and child elements. The bad news is that you'd have to wait for the IDEs and what not to support Maven 4... but hey, it's another reason to upgrade!
The colon delimited version is popular (you can see it in use with other build tools at, for example: http://mvnrepository.com/artifact/org.mockito/mockito-all/1.10.8 ) but not standard.