a) Grails had nothing to do with Spring. The reality is quite different, Grails is built on Spring, can leverage existing Spring beans and take advantage of all the Spring APIs
b) Grails requires you to leave Java and use Groovy. This was the most shocking one, most people thought to use Grails you have to only use Groovy. The reality is that if we wanted that we would have gone off and used Ruby on Rails.
The goal of Grails is to create a web platform where you can use Groovy for what its great at and use Java when it makes sense. Grails can completely leverage any existing Java codebase and existing Spring ApplicationContext definitions.
Another interesting event at the Spring Experience was the Web Development BOF on Friday night. The overwhelming cry from Spring users attending the BOF was for Spring to use more conventions on the web layer. Keith Donald asked me to give the audience an overview of the conventions Grails uses.
By the end of it the general audience response was: well why don't we just use Grails? Overall a good result :-)