bash you won't be able to login in the Terminal or via ssh. You could try
zsh instead but this is probably depending on the library as well.
Open Terminal, ignore the error message in the window, press Shift-Cmd-N and run
/bin/csh instead of bash. Then fix the symlinks.
Boot from the Install DVD, run Terminal from there and see whether version 8 of the library is still in
/usr/lib (or use libiconv.* from the Install DVD). If yes, fix the symlinks and reboot.
OTOH as OS X 10.8 ships with
libiconv.2 you might be out of luck:
lrwxr-xr-x 1 root wheel 16 Oct 16 20:50 libiconv.2.4.0.dylib@ -> libiconv.2.dylib
-r-xr-xr-x 1 root wheel 2113600 Oct 16 20:50 libiconv.2.dylib*
lrwxr-xr-x 1 root wheel 20 Oct 16 20:50 libiconv.dylib@ -> libiconv.2.4.0.dylib
Like option 1 but try to replace the MacPorts
/bin/bash with the original one. I don't know how MacPorts works but the original one might still hang around as
/bin/bash.orig or similar.
Reinstall the OS from the Install DVD (doesn't impact the rest) and re-apply the OS X upgrades.