They show up in the Calendar app at iCloud.com, or in Google's website calendar, and Yahoo's too.
I'm not aware of any app that will combine them on the iPad itself. Could happen. I believe that like calendar events, reminders can be accessed by third party apps, but I haven't heard of one yet.
On the other hand Pocket Informant used to work with the Calendar app, and it looks like they dropped that feature recently, probably temporarily. Maybe something in iOS 5 changed the way the info is shared.