What does this warning mean?

Some gdb versions are actually a bit more helpful with that warning. They output something like the following:

warning: .dynamic section for "libhello.so" is not at the expected address (wrong library or version mismatch?)

Make sure GDB finds the library your program was actually compiled with and that any debugging symbols, if in separate files, for the application and the library are current and consistent with the builds you are using.

An upgrade sounds like a good suspect for a cause. Prelinking libraries might also cause this warning to appear, although I think gdb can handle that case.