Routing w/ 2 external interfaces?

["arlen fletcher" <arlen.fletcher@att.net>]

I'm running an OBSD 2.9 machine w/ 2 external interfaces - call them xl0 and
xl1.

xl0 goes to ISP 1, and xl1 goes to ISP 2.  Obviously they're on different
networks.
I have ipnat mapping external addresses to internal server addresses, and I
have DNS servers at each ISP.

The idea was that if one ISP went down, inbound traffic would be routed to
the addresses provided by the ISP that was still up.

My question is: will OpenBSD know how to route the return packets out the
interface they came in on?  I'm concerned that my default route will be a
problem if it's pointing to the ISP that's down.

TIA!!!