Presto!
I just did a "default" install of Fedora 11 i386, then installed yum-presto, and applied all the already published Fedora 11 updates. Instead of downloading 121 megs of 66 updates, the system downloaded 10 megs of delta rpms and reconstructed the updates. That's a savings of 92%! When put in the context of going from 105 megs to 10 megs, 92% actually means something. This is very cool indeed, and given what we learn from publishing and consuming deltas within Fedora 11, we'll likely make delta rpm support enabled by default with Fedora 12. The future is small.