A search of Maven Central for the term "mq" (http://search.maven.org/) seems to show that the various MQ client jars (imq, imqbroker, imqjmx, etc.) haven't been updated since version 4.5.2 (April-June 2012).
Instead, newer versions of these jars only appear to be available via a distribution zip, "mq-distribution".
So far as I can see, this makes it impossible to declare simple dependencies in a project's pom.xml file.
For example, a project making use of the Java client API could previously have declared a simple dependency like so...
...but can no longer do so with versions newer than 4.5.2.
This also affects users of the embedded broker feature, as the new broker jars are unpublished.
Possible related issue: https://java.net/jira/browse/MQ-7