I've always wanted to be able to execute a show command while configuring a router (I'm never good at remembering subinterface numbers). A while ago Cisco introduced the do configuration command that allows you to execute any exec-level command (including telnetting to another device) without leaving the current configuration mode. For example, to view the interface numbers while trying to configure an interface, use the do show ip interface brief command:

router(config-if)#do show ip interfaces brief
Interface       IP-Address   OK? Method Status                Protocol
FastEthernet0/0 192.168.0.6  YES manual administratively down down
FastEthernet0/1 unassigned   YES unset  administratively down down
Serial0/0/0     unassigned   YES manual up                    up
Serial0/0/0.101 172.16.1.6   YES manual up                    up
Serial0/1/0     unassigned   YES unset  administratively down down
Serial0/1/1     unassigned   YES TFTP   administratively down down
Loopback0       172.16.0.22  YES manual up                    up