Reminders application does not open in Mountain Lion When I click on the Reminders icon in the dock it will not open.  

The console logs only one entry:

12/11/12 11:34:46,016 Reminders[6984]: NSConcreteMutableAttributedString addAttribute:value:range:: nil value


Does anyone have insight to this?  Is it hiding?
I had the same issue.  Try going to system preferences iCloud.  Turn off reminders off, press delete reminders from mac, then turn reminders back on and hit merge reminders from iCloud.  This solved my issue.  

Prior to this a did a system restore on the OS and it didn't do a thing still couldn't open reminders.