Yes, we have an open issue for that. We need to provide a compiled dlls against .NET 4.5, which won't compile with the Microsoft.CompilerServices.AsyncTargetingPack.Net4.dll.
The downside of the Microsoft Bcl Async package is that anyone that reference the RavenDB.Client package will have to also reference the Microsoft Bcl Async on his project, if they're using the .NET 4.0 of SL 5.0. The good thing is that this will probably won't be required when using .NET 4.5, but we are still looking into that.