The latest GWT Designer v7.2 build now includes beta support for GWT 2.0.
The initial beta is for Windows only. OSX and Linux support will be rolled out over the next couple of weeks.
Note that we will only be supporting the GWT 2.0 Java API initially. We have no short term plans to support the GWT 2.0 UiBinder as that is an XML-based API. GWT Designer is a Java GUI tool. It is not an XML tool. We plan to add support for XML and the UiBinder at some point in the future, but that will require several months of additional work as it requires a new parser, new editor and a great deal of additional infrastructure.