I hung a double flood/motion sensing on my MILs house at the 2nd story corner eave (much like you are wanting to do).  Not sure how high, but it must be around 20'.  Simple extension ladder for changing bulbs, which isn't great but works fine.  They sense motion at ground level just fine, and have day/night sensor so they stay off during the day.  Same basic function where you can leave them on by flicking the switch twice.
If you really want to, you can just get a separate motion sensor, and tie it to basic flood lights via X10 home automation, or hard wiring it.  The X10 stuff would be nice here as you could then control those lights from elsewhere in the house, like if you heard something and wanted to wake up whoever or whatever was rummaging around out there.
-Dave