(I’ve expanded this to include something else I’ve noticed)
First, for anyone who is upgrading to 10.8 – I had this issue when upgrading my MBP. Once you’ve finished downloading and run the installer for the first time, your Mac will reboot to install the update. I was getting a persistent kernel panic immediately upon reboot. The solution to this (for me) was to zap the PRAM – boot the machine and hold down Command-Option-P-R. After that the installer continued as normal.
Next, I noticed some issues with iCal and creating calendar events. I use Zimbra for calendaring and contact syncing between all my various devices (I’m loth to use iCloud for anything at all and Zimbra mostly Just Works with iOS and anything else I encounter).
Apparently the iCal in Mountain Lion changed just enough to break Zimbra, inasmuch as the defaults for event notifications seem to be “None”, and Zimbra doesn’t support this. This bug was recently fixed so we should see updated versions soon. In the meantime, however, you can work around this problem by going into Preferences->Alerts, selecting the account for the Zimbra server, and setting defaults instead of “None” for all the alert settings. After this things will work as normal.