It seems the API changes in 1.2 lead to a binary incompatibility with existing converters. In Mule (http://mule.codehaus.org/) we have a custom converter for ConcurrentHashMaps and the required constructor that takes a Mapper argument breaks with a ClassCastException when existing code uses getClassMapper() to retrieves a mapper for initialization. I wanted to avoid source changes (from getClassMapper() to getMapper()) to allow running with 1.1.3 for a limited time.

It seems to me this should this be possible, otherwise the whole point of providing @deprecated method stubs would be moot. You can look at the code:
http://fisheye.codehaus.org/browse/mule/trunk/mule/core/src/main/java/org/mule/util/XStreamFactory.java?r=3191
around line#50.

It would be nice if this could be addressed in 1.2.1, otherwise we'll have to stick with 1.1.3 for a while :/