I'd read the tech notes and as far I was aware the policy would not apply deletions to calendar entries and tasks.
(This had always been the case previously)
Well it turns out Microsoft changed this with Exchange 2010 SP2 Rollup 4 and above...
Now, if you set the tag type as 'All other folders in the mailbox' IT WILL apply to calendar entries and tasks!
If you don't want this to happen, you need to modify the registry on the Exchange server as followed before applying the Exchange 2010 SP2 Rollup 4 or above:
(0= Do not process Calendar and Task folders)
(1=Apply to Calendar and Task folders)
In my opinion with the rollup updates Microsoft should have set retention for Calendar and Tasks to disabled by default and allowed Administrators to enable it if required. Instead they've arguably done it the wrong way round, catching many other people out too. I'm just thankful I didn't set the tag to permanently delete!
*** Update ***
Because the above tag was applied to 'All other folders in the mailbox', the retention policy still runs against Notes, Drafts, Outbook, Journal, Conversation History etc. Again this caught me out, as because as they are listed individually in the dropdown for tag types, I thought they would be treated seperatley.
If you don't want the retention to run against these item types, create a new disabled tag.
i.e, to prevent notes being deleted, I created a tag called 'Never Delete Notes'
I chose 'Notes' from the tag type dropdown
I then selected 'Disable this tag (items are never expired, moved or deleted'
Finally I applied this tag to the retention policy.