Did you let it install the Software Update update (with the new package signing stuff) by any chance? I know when I did a re-install at the weekend it tried to force this on me. I did it first time and Software Update showed me as up-to-date, plus it wouldn't allow me to download the 10.1.5 upgrade combo package.
So I re-installed again, *didn't* install the Software Update update, ran the 10.1.5 upgrade combo patch (it's big - about 45MB - and can be found on the Apple support site), *then* let Software Update do its thing, and all was well.