Having the same issue here with Ubuntu 11.04 also (32 bit), I used to build the same software on my Ubuntu 11.04 build VM and then run it on a set of target Ubuntu 11.04 machines and everything had been fine until now.
The target machine is now coming up with the following messages when I run our software:
I have updated both machines, but still getting the same issue, I am still trying to figure out why this is happening, as I don't use any PPA's on these machines.
/usr/lib/i386-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.15' not found
Both the build machine and target machine have the exact same version of libstdc++6 installed and libstdc++6-4.5-dev so this shouldn't be happening.