libstdc++-v3 stability?

[Stan Shebs]

For a long time, Darwin (aka Mac OS X) has stuck to static linking
for libstdc++, mainly because the C++ ABI was changing regularly.
But now that things are more stable, we can save some space in apps
by making libstdc++-v3 a shared library.

My concern however is that while the C++ ABI may be stable, what
about the libstdc++ ABI?  It seems like it would only take a small
tweak to the signature of an app-visible method, and then an old
app would not be able to dynamically link to a newer libstdc++
dylib, which means that you've have to keep around a copy of the
previous libstdc++.  Darwin does have a dylib versioning mechanism,
but I'd hate to have millions of iMac disks filled up with dozens
of libstdc++ versions 1/2 :-)  (Yes, I know Microsoft does just
that with DLLs, but I'd like to do better.)

So, how many binary-incompatible versions of libstdc++ do people
expect to see over the next few years?

Stan