From what we can tell.. we are having this exact issue. Distribution lists, or having a distribution list included amongst other invitees not on a DL are having this issue. If you send to multiple users not on a DL, and don’t include a DL at all, everything works just fine.
Are you on Office365 as well?
Our user had a non-response required invite sent to them, deleted the item from the iPhone, then everyone that was included on the invite got 2 "informational" notices of cancellation from him directly.
We are all hoping this thread picks up steam, as this issue along with the “Hijack/Cancellation iOS 6.0.0 issue” is, and continues to be a major issue in our enterprise.
Yes, we too are on Office 365 for corporate e-mal so options to run other types of clean-up jobs on the Exchange server or for us to patch our Exchange server.
Our testing for meetings where no one is on a distribution list seems to work regularly without issue. Unfortunately, the users that do get impacted the most are our leadership teams, as they are most likely to have delegates (admins) for e-mail, be invited by distribution list, most likely to be leveraging an iOS device whether iPhone, or iPad, or many times they have both. So when issues such as the new iOS calendar issues occur, the occur in the most visibile way possible.
iOS is our corporate standard for mobile, but continues to be challenging given the continuing issues. This isn't the first time we have experienced issues with new iOS releases and failure to work within the enterprise.
Also not happy that Microsoft doesn't secure ActiveSync. I don't understand the protocol limitation as to why Microsoft refuses to fix on their end as well, but I can't think of a design reason why a user's response would be allowed to assert ownership of a meeting assuming the meetings are "owned" by a central store such as Exchange server. Regardless the first issue still lies with iOS not behaving nicely.