Scavenger's [answer](http://meta.stackoverflow.com/questions/981/syntax-highlighting-language-hints/43775#43775) is a good idea, but `#` might be something that is valid in the code like a pre-processor command in `C++` that can be part of the actual code sample.

I suggest to have something that is specified _before_ the code block in a special way. The code block is indented with 4 spaces, so the language specifier could be indented 3 spaces (and only mean something when appearing immediately before a code block). This follows the simplicity in the markup rules that already exists.

>   VB.NET  
>     Dim myVariable As String 'Here is the first code line  
>     myVariable = "Hello world"