I don't have an ACS1121 handy but I believe the ADE-OS appliances have a common base.
Here the output from one running Cisco Prime LMS 4.2.1 (the latest version of that product):
[*****/root-ade ~]# cat /proc/version
Linux version 2.6.18-238.1.1.el5 (mockbuild@x86-008.build.bos.redhat.com) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-50)) #1 SMP Tue Jan 4 13:32:19 EST 2011
