I don't think yum-presto should be enabled by default in Fedora 12.
There are two bugs that really should be fixed before that can happen:
https://bugzilla.redhat.com/show_bug.cgi?id=524720
https://bugzilla.redhat.com/show_bug.cgi?id=524982
The first one has to do with the md5 checksum error mentioned earlier on
this list.
The second one has to do with the fact that when rebuilding the rpms, we
have to recompress the data, and xz compression is over 10x slower than
gzip.
Jonathan