As with most Gnome mockups, this looks very nice, and very intuitive. I can't wait to see Gnome mobile come to fruition.
The only thing that I'm a bit iffy about is the Location entry being above the date/time entry, essentially giving it more emphasis/focus.
Maybe I'm weird but I don't put locations in most of my calendar reminders, but I almost always add a specific time, and to me it seems like the Location entry should be immediately below that.