AKU's answer should help you out:
.NET namespaces
He links to Microsoft's guidelines:
http://msdn.microsoft.com/en-us/library/893ke618(VS.71).aspx
You should consider reading the the rest of the guidelines starting here:
http://msdn.microsoft.com/en-us/library/czefa0ke(VS.71).aspx
The remainder of the post is also very informative:
.NET namespaces
In your case you would go with:
CompanyName.ProductName
CompanyName.ProductName.ClassName
CompanyName.ClassName.IsUpperCase(string str)
The .NET guidelines don't follow the Java style of using reversed FQ domain names to specify namespaces, and I've yet to see a commercial component such as Telerik or Infragistics for example follow anything other the guidelines than the MS ones.