Repro steps:

Start mongos with the --configdb parameter referring to a mongod that doesn't exist:

Fri Jan 28 17:51:17 mongos db version v1.8.0-rc0-pre-, pdfile version 4.5 starting (--help for usage)
Fri Jan 28 17:51:17 git version: 7fa627b35627e94c94aafe9b0275de665129acd2
Fri Jan 28 17:51:17 sys info: Linux behackett-dt 2.6.36-gentoo-r5 #4 SMP Mon Jan 24 10:24:02 PST 2011 x86_64 BOOST_LIB_VERSION=1_42
terminate called after throwing an instance of 'mongo::SocketException'
what(): socket exception
Aborted

I can also reproduce this by passing a configdb that hasn't completely settled down yet. Logging mongos to a file you can see something like this:

Fri Jan 28 17:41:26 mongos db version v1.8.0-rc0-pre-, pdfile version 4.5 starting (--help for usage)
Fri Jan 28 17:41:26 git version: 7fa627b35627e94c94aafe9b0275de665129acd2
Fri Jan 28 17:41:26 sys info: Linux behackett-dt 2.6.36-gentoo-r5 #4 SMP Mon Jan 24 10:24:02 PST 2011 x86_64 BOOST_LIB_VERSION=1_42
Received signal 6
Backtrace: 0x50ba14 0x7f6f3609d1e0 0x7f6f3609d165 0x7f6f3609e580 0x7f6f378b65d5 0x7f6f378b4a06 0x7f6f378b4a33 0x7f6f378b4b2e 0x51046e 0x510819 0x5a70dd 0x61736b 0x611e38 0x7f6f36089bbd 0x48ecb9
mongos(_ZN5mongo17printStackAndExitEi+0x64)[0x50ba14]
/lib/libc.so.6(+0x321e0)[0x7f6f3609d1e0]
/lib/libc.so.6(gsignal+0x35)[0x7f6f3609d165]
/lib/libc.so.6(abort+0x180)[0x7f6f3609e580]
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/libstdc++.so.6(ZN9gnu_cxx27_verbose_terminate_handlerEv+0x115)[0x7f6f378b65d5]
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/libstdc++.so.6(+0xcaa06)[0x7f6f378b4a06]
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/libstdc++.so.6(+0xcaa33)[0x7f6f378b4a33]
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/libstdc++.so.6(+0xcab2e)[0x7f6f378b4b2e]
mongos(_ZN5mongo16DBConnectionPool3getERKSs+0x34e)[0x51046e]
mongos(_ZN5mongo18ScopedDbConnectionC1ERKNS_5ShardE+0x89)[0x510819]
mongos(_ZN5mongo12ConfigServer15dbConfigVersionEv+0x1d)[0x5a70dd]
mongos(_ZN5mongo12ConfigServer18checkConfigVersionEb+0x1b)[0x61736b]
mongos(main+0x1648)[0x611e38]
/lib/libc.so.6(__libc_start_main+0xfd)[0x7f6f36089bbd]
mongos[0x48ecb9]
===