I was disappointed to find that Calendar and Reminders are two separate apps in Mountain Lion (OS X 10.8.3). But I am more disappointed that I didn't think to check for discussions about this issue before upgrading. I didn't think to check on iCal in Mountain Lion because iCal was working so well. If it ain't broke why change it?
I have a little network of three 2011 model iMacs that had been running Snow Leopard (OS X 10.6.8) with Snow Leopard Server on a Mac mini. We don't share files as much as diary entries and our to-do lists. We are very happy with how OS X 10.6 handles this.
There are a couple of pieces of software that I would like to use that only run on Lion or Mountain Lion so I thought I'd upgrade one of the iMacs to Mountain Lion. After the upgrade Calendar appears to have picked up the iCal data on the iMac and our shared iCal data on the server but not the to-do list items. It appears from the discussions that no provision has been made to import the data into Reminders. Strangely, some completed to-do items appeared in Reminders.
After my first shutdown and next startup of the iMac I also encountered the frozen dialogue box "Upgrading calendars …" which some of the discussions mentioned. Calendar quit OK and I re-opened it without the dialogue box appearing again. Calendar is a big window too that doesn't seem to be able to be reduced down much.
Some years ago I used an application called Now Up-to-Date on various Macs running OS 8. It combined a calendar and to-do list that could be shared on a network but the network features didn't work with OS X in Classic mode. It wasn't until later versions of OS X that iCal incorporated the combined features.
This is a bit of a long story but for the first time using Macs I think I will uninstall an upgrade. My little network was working fine. I'm also not keen to upgrade the server to Mountain Lion and risk messing up features that have been working well.
Thanks so much for telling me about 'Reminders'. I used to use it in Lion for to-do & to-bill lists and I broke into a cold sweat when I couldn't find it in Mountain Lion. Why on Earth does Apple change things that work perfectly well? And the iCal graphics are hokey - pretend tear-off wall calendar. Bring back Leopard! And why doesn't Keycahin synchronise through iCloud??! I'd better make myself a cup 'a' tea.