Hi,
While executing my application I am getting the error:
ld.so.1: testapp: fatal: libc.so.1: version 'SUNW_1.22' not found (required by file testapp)
ld.so.1: testapp: libc.so.1: open failed: No such file or directory.

I suppose it requires version of libc newer than which is on my system. But, how do I know which version it requires?
I copied the executable from some other system.

Also, by mistake I renamed libc.so to some other name and now no command is working now. Is there any way you can think of renaming it back to the original name?

Regards,
Girish

Correct.
ldd -v binary
This won't work. The C library is tight to the O/S. Compatibility is guaranteed to work, but only if a newer library is found.
Boot on a CD or DVD and rename back libc.

Interesting. I get the exact same error message when trying to run the "usermod" command, but everything else works fine... Is there some sort of libc patch available? Doesn't make sense that a command that comes with the OS isn't compatible with the OS!

So what O/S version are your running ?
Check what libraries your binary is requesting like this (that one is better than the previously suggested ldd):
Check what the installed libc offer:

"Boot on a CD or DVD and rename back libc."

But, "mv" command itself wont work even after I boot on a CD.

Why won't it ?
The point in booting on a CD is to have a working O/S available. A Boot CD is technically a live distro so all commands available on the CD are expected to work.
Please elaborate if you experiented differently.

bash-3.00$ cat /etc/release
Solaris 10 1/06 s10x_u1wos_19a X86
Copyright 2005 Sun Microsystems, Inc. All Rights Reserved.
Use is subject to license terms.
Assembled 07 December 2005

bash-3.00$ pvs /usr/sbin/usermod
libsecdb.so.1 (SUNW_1.1);
libproject.so.1 (SUNW_1.1);
libtsol.so.2 (SUNW_2.1);
libc.so.1 (SUNW_1.22.2, SUNWprivate_1.1);

bash-3.00$ pvs /usr/lib/libc.so.1
libc.so.1;
SUNW_1.22.1;
SUNW_1.22;
SUNW_1.21.3;
SUNW_1.21.2;
SUNW_1.21.1;
SUNW_1.21;
SUNW_1.20.4;
SUNW_1.20.1;
SUNW_1.20;
SUNW_1.19;
SUNW_1.18.1;
SUNW_1.18;
SUNW_1.17;
SUNW_1.16;
SUNW_1.15;
SUNW_1.14;
SUNW_1.13;
SUNW_1.12;
SUNW_1.11;
SUNW_1.10;
SUNW_1.9;
SUNW_1.8;
SUNW_1.7;
SUNW_1.6;
SUNW_1.5;
SUNW_1.4;
SUNW_1.3;
SUNW_1.2;
SUNW_1.1;
SUNW_0.9;
SUNW_0.8;
SUNW_0.7;
SYSVABI_1.3;
SUNWprivate_1.1;

Well, that explains the immediate "why" of the error message... However, it doesn't explain how things ended up this way to begin with! Interesting...

Yes. I tried it by using bootable CD but "mv" command itself was asking for libc.

Okay, that's a known issue due to a discrepancy in 120051-03 or 120051-04.
It is fixed with this patch: 120051-05.

Ugh. It appears that useradd is also broken by this flaw, and I now have a new user to add! 120051-05 requires that 118855-25 (or newer) be installed. Reading the horror stories on SunSolve makes me very afraid to even attempt the 118855 patch (despite having a successful install on a test box)! Any other workarounds for adding users?

You can use smc to create a user account, or you can simply edit /etc/passwd, run pwconv, set a password and create/chown the home directory.

I may have to edit /etc/passwd then. SMC is also broken (most likely due to some patch awhile back).