When I try to create a wcf client with the ClientBase<TChannel> class, the InnerChannel throws an exception. The same code worked in sl4 and sl5 beta...
After investigation, It appears to be caused by the 'Require elevated trust' option. When it is check it fails.
See those 2 posts in the Silverlight forum. The first one is mine, the second is I think someone who has the same problem.