Can I use a different linux distribution in an LXC container?
It is possible to run a different distribution, but the kernel that's being used, has to be the same.
So if your Debian and ubuntu use the same kernel or can work with the same kernel, there shouldn't be a problem. I don't know however if 12.04 can support the latest lenny kernel (it's pretty dated and support has been dropped for lenny by debian).