I am currently picking up Grails and Roo.
The vast majority of training materials appear aimed at new developers or existing Java developers.
Does anyone know of any guides, resources or tips for translating existing development experience with .Net (C#/ASP.Net/Asp.Net MVC)?
Not that I know of, but there may be resources for .Net -> Java, which is probably all you really need. Perhaps tag your question with 'java' as well?
BTW, it's probably worth mentioning which language you're familiar with on .NET.
Have you done much ASP MVC? From the code my friend has shown me it looks very simlar to Grails. Grails is VERY easy to use so I would suggest learning that first. I have not learnt Roo as it looks more complicated and I can achieve all I need with Grails very quickly. Hope this helps a little.
ASP.NET MVC and Grails both originated from Ruby on Rails - so developing principles are similar for them. I would recomend you to start learning Grails with this book. However getting used to dynamic Groovy after static C# may take some time.