Well, if you haven't tried it already, and you're interested in PDO, don't: just grab the latest snapshot from snaps instead.
Lack of communication, and perhaps some politics, meant that 5.1beta shipped with a stupid bug in the PDO core extension, causing a infinite loop on request shutdown.
I'm somewhat annoyed at this, because it's a showstopper that we could and should have caught before announcing the beta.
Oh well, such is life.
I'm just getting stuck into packing things up ready to move into our first home in the USA (we've been living in an apartment for a few months), so I'm not going to let it bother me.
Have a good weekend.