It's taken me awhile to figure out why NATting GRE wasn't working. 
Then I remembered: I have IPSec VPNs running - something that uses GRE in my VTI configs!
So I am wondering: can I have certain GRE traffic from certain source IPs go to LOCAL land and act as a proper VPN, yet have different source IPs not get handed off to LOCAL land and get NATted to an INSIDE IP instead?