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A suggestion for anyone looking to expand on this, would be to build something in grails and play and compare. I prefer grails just because of the groovy language, but this framework does look promising

... can a project be deployed as a war file to Tomcat now? Remember looking at it way back when and that really wasn't an option (if I remember correctly that is).

Yes it can using the play war command. A project can also be deployed to GAE using the play gae:deploy command using the latest builds

Play is awesome. It is fun to develop, it is well thought out into most details and I switched to it from Grails and I am not looking back.

Nearly everything about Grails is implemented in a half-assed manner. It works for simple use-cases, but when you take it for a spin for your more complex domain, it quickly fails, because it was intended to work for simple "Book-Author" sample applications. Most of the documentation is based around those type of apps. Got anything more complicated? Good luck!

I could say that I switch to play because its hot-reloading is more robust, because it starts apps faster or because writing tests is a lot easier and faster to run. But those issues are not my motivation, it basically boils down to the half-assed-ness of Grails. That is the main reason why I am switching from Grails to Play. The half-assed nature of your tools will have a huge impact on the product you build. I can no longer accept waiting 1-2 years for issues to be fixed and having so many dumb workarounds for things that should just work out of the box. I dont wanna get excited again for new features just to debug them for 2 hours and afterwards find n JIRA entries for my use case.

you concerns about a complex domain/project, I have to disagree about. I've created a pretty complex project using grails, and the entire experience was actually really good, compared to doing the standard java way using the same tools of spring mvc, spring, hibernate. I can't compare it to play, but using the latest 1.2 release of grails, I haven't come across this half assed-ness you're talking about. I imagine you would have issues with rails in this case as well. Maybe you can expand on what you feel is lacking in the grails framework and how play improves on it, with examples.

Play! and Grails share the same ideas and concepts. The biggest difference is that Play! is pure Java and does not rely on the servlet API. While Grails uses Spring under the hood, Play! does not delegate to a IoC framework. However, Play! of course delegates validations, persistence, etc to the appropriate libraries and make sure that error reporting is adjusted (Hibernate errors are improved for example).
I think Grails is a really good framework as well. It seems to me that Play! is more accessible because you stay in pure Java territory.

How do you pronounce "Play!". Do you simply say "play", or do you really have to shout a little when saying it?

You pronounce it exactly as if you were a kid totally exited by the idea of playing your favorite game ;) So there is a subtile intonation at the end indicating you are going to have a lot of fun.