We are moments away from posting Second Life 1.9.0(17) as an optional viewer update. Unfortunately, there has been a delay in the creation of the Mac UB build for this new version. The update is optional, meaning you won't be locked out of SL...but you won't get the latest version right now, either.
PLEASE do not select "Download" when prompted to update. Watch this space for more info on a 1.9.0(17) Mac UB beta in the next day or two.
Thanks!