Visit the play store and you will see that the latest version of Google Plus is only available to Gingerbread devices. The cause is an error in the manifest file and should be fixed soon, so hang tight. Nonetheless, this is sure to cause pandemonium across the interwebs.
Vic Gundotra, senior vice president of engineering at Google released this statement on the matter. See his Google Plus feed here for more.
Folks - looks we have an issue with our manifest file... We are fixing right now. Sorry about the incompatibility errors.