There are some problems with Arrays of DTO's between seam 2.1-snapshot (4/25/2008) and GWT 1.5m2 remoting/rpc.
i.e. SimpleDTO getMyDtoArray()
It is the non-informative 'IncompatibleRemoteServiceException' error and mentions the application out of date, which it is not. No sample project or test case yet as mentioned in the forum post just identified the source of some problems.