This may impact many people because it is a change in the validation of the parameters of this method. The method is defined below:
RegisterClientScriptBlock(string key, string script);
In .Net 1.1, the key parameter could not be null, but it COULD be a zero-length string. In .Net 2.0 Beta 1, this parameter is validated so that an empty string is not valid input. It throws an exception.
Maybe a validation change for the better, but a breaking change none-the-less.