I'm obviously biased in this area

so it's a good thing Gregg is here to provide some balance and wisdom.
As Gregg said, technologies do come and go and some last longer than others. Usually they follow some sort of curve. I think that Groovy and Grails are still in the upward swing of that curve. If you look at the way the tool vendors are competing with each other to provide support and if you look at the job trends on indeed.com, you get the impression that the interest in and adoption of these technologies are still growing.
There will be other new languages and frameworks and that will continue to make our industry exciting, but if you're wondering if Grails will be around long enough to make it worth learning, career-wise, I think the answer is a resounding yes!
Dave