There is a strange issue with VB.NET/VS 2003. The Page.RegisterClientScriptBlock method is not visible through intellisense.
It was very weird as the same method was visible in a C# project. Finally Googling on this issue I found out that by default VB.NET projects need to be configured to show "Advanced Members". To set the same, go to:
Tools->Options->Text Editor->Basic->General
and then uncheck this option: Hide Advanced Members
I wonder why MS did that!