Kalpesh:
"Back in the day" one could roll back an iPod to a previous version because of how Apple controlled updates. Later on, Apple decided to change the process. That change makes it impossible to roll back to a previous version of iOS.
Basically, Apple forces iTunes to "phone home," and request permission to update your device. At that point Apple's update server looks at your device type, device id, and iOS version. If you are running an older version of iOS, and if your hardware is supported by the latest version, then iTunes is allowed to update your device. Roll back are not permitted. Period, end of story....
So then you have two choices:
1. Do not update your iPod to 5.1.1
2. Have the app written for 5.1.1 and then upgrade your iPod.
Oh, and if you still have doubts, let your mouse cursor do the walking and search the archives... And Google will no doubt find TONS of info about this...