@Themathers: actually, I have successfully installed Viber before, then used AppBackup and Restore to restore it to sdcard.
Then, I flashed CM7 using CWM, trying to use File Manager to install Viber from sdcard (.apk) failed with just this message "Application is not installed" - no more details given.
@TheManii: with stock 408 ROM, I could install it. Only after switching to CM7 I do not have that ability.
Michael, you're using CM7, aren't you. If that is the case I guess that there might be some issues with my CM7. Have you installed GApps? I do not install (just install Vending, GoogleServicesFramework and without Calendar Sync, other apps...) so it might be the case. I will try flashing CM7 again, and install all GApps to see whether it works!
I am using CM7, and loaded the full GApps from the same thread as the CM7 link.