I have not found any DT number. But I can tell you that the fix consists of a patched version of NBES.EXE. It was named a quick fix - may be this is not publicly available.
The root cause for the issue is that the UID field in the ICS file is not reused when processed by BES. As of RFC 2445 the UID is the unique key for the calendar entry and therefore the other application (Exchange, GoogleMail,...) can not find the corresponding calendar entry.
You can rebuild the scenario quite easily with GoogleMail as you have direct access to the ICS attachments there. When you compare the ICS attachment when accepting/reject on BlackBerry vs. Notes, you will find that a new UID is created whenever you do that on the device. It was quite easy to convince RIM that that is a bug in BES (and not in Domino and not in CoExistence solution ...)