Unless the iOS notification center violates a patent, what exactly would Google be suing over? Copying ideas isn't necessarily against the law.
Google has filed a Patent Application for its notification center. I don't know if a Patent has been granted on it yet. I took a look at the Application at Google's site, which makes for some interesting reading - because basically they are trying to describe information popping up in little boxes around the perimeter of a display screen. Something thats been happening for quite some time now.
I honestly don't know if Google's application really is patent-worthy. Maybe someone with a lot more time on their hands can go through it and explain what, exactly, is special about it.
But even if it is granted a patent, it would most likely be something Apple (or any other competent software engineer) could easily work around.
If nothing else, it illustrates how difficult it is to describe software patents; how complicated it must be for the Patent Examiners who look at them; and why so many Software patents get invalidated in court. Which is not to say they ought to be outlawed.