After being a little bit disappointed about using Grails in Alfresco (see my post below), I decided to see how could Alfresco be integrated with Groovy.
I took the groovy.servlet.GroovyServlet implementation and added a few lines that allows to configure a Spring bean as servlet parameters and to make them available in the groovy scripts.
The code is not yet published somewhere as a forge project but you can find it here. All you have to do is have Spring and Spring MVC on your classpath (Alfresco already has it) add Groovy libraries (groovy-all is a good solution, at least version 1.5.x) and configure your web.xml with the Groovy servlet that I am providing, an example could be found here. That example is using spring's DispatcherServlet but you could use the GroovyServlet as any other servlet, directly in your web.xml. If your are also using the DispatcherServlet then you have to configure it correctly as shown here.
Once you are done with those configuration just add the desired beans to the servlet configuration and you are on "rails" with Alfresco.
The samples provided here are using the ServiceRegistry bean and it is mapped in Groovy as registry, so to get any other Alfresco service you have to start from there. But you are free to map whatever bean you want.
Although, some people are saying that there are problems using Alfresco beans with Groovy, I did not have those problems till now, but I am not saying it will happen.