I have deployed Grails 1.3 apps to WebSphere in the past and there were a few hiccups, but nothing that couldn't be fixed (
http://grails.org/Deployment#Websphere 6.1). That was a long time ago and I can't think of any reason why it wouldn't work on the latest WebSphere 8.
The only thing you could run into is if WebSphere includes a duplicate Jar but different version of something that's packaged with Grails. I always configured WebSphere to be PARENT_LAST anyway with any Spring app to avoid those classloader issues. PARENT_LAST allows an application classloader to override and provide its own version of a class that exists in the parent classloader.
I'd say give it a try and see if you run into any problems. If you do, just post the exceptions / deployment errors to the mailing list so we can help get them resolved.
Thanks,
Bobby