Show Ver will display the Serial number on 2600's as well (may depend on IOS version)...Also if you need to find the Serial number of a neighboring router or sqitch, "show cdp neighbors" should work.
sho version
Cisco Internetwork Operating System Software
IOS (tm) C2600 Software (C2600-I-M), Version 12.1(2), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2000 by cisco Systems, Inc.
Compiled Tue 09-May-00 23:34 by linda
Image text-base: 0x80008088, data-base: 0x807D2544
ROM: System Bootstrap, Version 12.1(3r)T2, RELEASE SOFTWARE (fc1)
xxxxxxx uptime is 1 year, 51 weeks, 4 hours, 49 minutes
System returned to ROM by power-on
System restarted at 17:46:01 UTC Tue Apr 3 2001
System image file is "flash:c2600-i-mz_121-2.bin"
cisco 2621 (MPC860) processor (revision 0x200) with 53248K/12288K bytes of memory.
Processor board ID JAD05041H47 (2131750795)
M860 processor: part number 0, mask 49
Bridging software.
X.25 software, Version 3.0.0.
2 FastEthernet/IEEE 802.3 interface(s)
2 Serial network interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read/Write)
Configuration register is 0x2102