Major Description
I was glad to see the ongoing work for a more modular build (thanks Scott Carey!). Whilst looking into the cross-platform IPC facilities for use in OSGi I noticed something that makes OSGi compatibility (and maintenance) more difficult than necessary, for no good reason. I plan to submit OSGi bundle patches later (though not necessarily for the 1.5.0 release) so this is a necessary prelude.
The term "split packages" refers to the situation that two artifacts carry the same packages, which means that the classes in both packages are more or less randomly munged together at runtime. This unfortunate situation is "mostly" without consequence in "normal" flat-classpath Java (assuming there are no overlaps!), but bad for OSGi since class visibility & wiring is based on package visibility. Split packages generally make any form of automatic package resolution (for deployment) almost impossible.
As far as I can see there are several classes in packages across artifacts that can easily be moved a bit without really disturbing anything. Some examples:
org.apache.avro.specific is defined by acro, compiler AND ipc
org.apache.avro.ipc 
is defined in avro and contains classes that could go into avro:avro.io (the buffers) or avro-ipc:org.apache.avro.ipc
It seems that the previously unmodular package membership of classes has been carried over during the artifact separation. I'd like to see this cleaned up as well before the 1.5.0 release, as this is a breaking change. However, most of the overlaps can be fixed easily with IDE refactorings like package renaming or by moving classes.
Please let me know if this is an acceptable change and if you want me to provide help/patches etc.