While Groovy will give new energy to the entire Spring portfolio, Grails, specifically, will help round out their competitive position vis a vis RedHat. In recent demos at our Java User Group and at JavaOne, it appears that RedHat is using Seam to draw people into their whole platform. And until now, SpringSource hasn't had a direct answer to that. Not only will they have a framework that competes with Seam, but they will have one that blows it out of the water! Don't get me wrong, Seam is heading in the right direction and with the eventual advent of JSF 2.0 it will get even better, but it is no where near as productive, effective or awesome as Grails.
Over the next several days, we will probably be hearing a lot about what this acquisition means to the future of Groovy and Grails. I think, overall, it will be a major positive in that regard. But, I think the real winner is SpringSource. Congratulations to Rod Johnson and the gang for once again jumping ahead of the curve!