In C# 3 and Visual Basic 9, you can declare variables without having to specify the type of the variable. This isn't the same as the variant type in Visual Basic, but rather the compiler's ability to determine the type of the variable from the initialization value.
Here's how it works:
var i = 3; //C#
Dim i = 3 'VB
Using implicit declaration of variables requires you to specify a non-null value since a null only represents the generic object type.
EDIT: You can get the implicit declarations feature in LINQ projects (CTP) with Visual Studio 2005.