I've just ran out of IP adresses in my outside network segment. I'll have to configure a new outside vlan in the same context in order to assign new static translations for new servers in my inside network. I'm currently using a default route pointing to the outside interface.
My question is... what happens with routing to the outside once I configure the second outside interface? I figure that connections initiated from the outside (any of the two interfaces) will work ok. What about any connection attempted from the inside?
No you don't need a secondary IP address.
There is no default-gateway for the subnet because it is not actually attached to anything ie. it is just used for NAT.
So lets say you pick a new subnet of 192.168.7.0/24. All you have to do is make sure that any traffic destined for 192.168.7.x is sent to the outside interface of your FWSM. How you do this depends on your topology but typically it would be done with static route on MSFC redistributed into your internal routing protocol. So on MSFC
ip route 192.168.7.0 255.255.255.0
then as stated you need to ensure this route is propogated throughout your network or at least to the parts of the network that need access to these servers.
Then on your FWSM
static (inside,outside) 192.168.7.x netmask 255.255.255.255
and add access rules to your existing acl that is on the outside interface eg.
access-list outside_in permit tcp any host 192.168.7.x eq 80