Here we go.
First of all, you can't update on the device itself. It shows you the new hangouts with no option to install. Even while it says "replaces GTalk), old GTalk is still on the tablet (tried P1000). When I go to the web-interface of the play store, I can choose to install Hangouts. Did that and guess what:
Meanwhile Google acknowledged that problem and writes within the app description: "NOTE: If you aren't yet able to install Hangouts on your phone or tablet, or the Play Store "Open" button takes you to the old Talk app, hang tight, we are rolling out Hangouts to everyone over the course of several days. Rest assured Hangouts will be available soon for all Android 2.3+ devices, tablets included!"
Well it can happen. Seems we just have to wait a bit longer. I'm happy that it started yesterday at all. Didn't see that coming so fast. The Chrome App and the integrated New Hangouts in Google+ are already there.