The need is as the following.
I use the AXP as a remote network monitoring system. I want it collect the NM data through the eth0 and I will connect the eth1 to the DMZ. So the NAT let me access the AXP from Internet. So I should use the eth1 as the default gateway interface. The topo as the following.

|-------------|
|   Internet   |
|-------------|

  |

|-------------|
|     FW       |
|-------------|

                                                  DMZ  |
  |eth1
|-------------|
|     AXP      |
|-------------|
  eth0  |
 |

|-------------|
| Internal Net|
|-------------|
If it can support the default-gateway on eth1 that't great. If it can not I can only use one more router ethernet interface as the unnumbered interface for AXP to connect NAT while the GE on the AXP is not used.
I think this kind of need is there in many customer environments. Can Cisco support it in the future?