It's btrfs-as-default that's deferred. We've had it available as an
option via interactive anaconda for many releases now (for a while it
required a Sekrit kernel parameter to prove you were One Of The
Initiated, but that was dropped a while back).
btrfs is not available as a target file system during installation. This
is a temporary situation and will be resolved in Fedora 18. btrfs is
still available after installation.
The reason is that anaconda's *backend* code for dealing with btrfs was
rewritten early in F17 cycle to be much more sophisticated and take
advantage of all btrfs' shiny features. At that time, it was still
expected that the new anaconda UI would land for F17, so the frontend
code in the old UI was not updated to cope with the new backend code.
Unfortunately, newui got deferred to F18, so now we're stuck in F17 with
some awesome backend code but no matching frontend code. No-one has time
to substantially rewrite the oldui partitioning code just for btrfs, so
it's been taken out from the UI entirely just for this release.
You can still use btrfs during installation in F17 but only if you use a
kickstart. Happily, upgrading installations with btrfs partitions works
(Side thought on the release notes for the documentation team - it seems
a bit odd that this is in the 'notes for system administrators' section
when sections exist for both 'file systems' and 'installer', either of
which seems more appropriate for a note about use of a file system in
the installer. :> Also, it would be good if the note explained that you
can use btrfs via kickstart.)