Darren, I vaguely remember that syncing settings are an opt-out, so if you re-installed Google Apps after the update, you would have to actively de-activate this.
However, for me, this does not yet fully explain what went wrong. If the syncing was set in the last OS version as well, you should be able to see calender entries (also via the web interface). If it wasn’t, and the (online) calender would be blank, but should IMHO not overwrite your calender entries if they were preserved during the update. Please check your calender online, because as far as I can see, that’s the only option where you could still possibly get your entries back.
Since Stefan still has his calender after the update, and without using GApps, I see several possible explanations, but none of them helps you regaining your calender, sadly.
A likely explanation would be that an update of Android (always) deletes your calender entries, and most people don’t notice because they either have a local backup or store their stuff in the cloud. This would mean Stefan lost his calender entries with the update but forgot about, because he restored them either from a backup or from his Owncloud (I think he uses this to sync his calender(s), as I take from another thread). It could also mean that his calender entries are stored on the SD, or at least somewhere out of the basic android partition, and did not get deleted because of this.
We could try to rule out that stuff if people come forward who upgraded to 1.6, applied the unified partitioning, and do use Google Apps without ever having the syncing active, and their calender was left intact. (Or if someone knows about how Android implants the calender storage. )
If they loose the calender then when then upgrading to 1.8, that would explain something, at least. Not that it helps.
Bottom line: backup everything, constantly, and especially before you do apply system software updates. Everything else is like searching needles in haystacks. :-/