Oracle's contributions to DTRACE are funneled upstream to http://dtrace.org, as is the norm with open-source projects. At present, there are no public-facing repositories at Oracle for this.
Totally incorrect, see entry from Elena below.
Edited by: TommyReynolds on 14-Dec-2012 11:17
The statement above is incorrect.
We are not adding our contributions to dtrace.org at the moment.
The locations of the public portions of the code are listed here: https://oss.oracle.com/projects/DTrace/source.html (git repositories)