It looks like you can hold a reference to the method you are entering and exiting by using the MethodExecutionTag.
You can assign what you want to it as it is just an object and is bound to the eventArgs of the enter and exit events.
This means you could also make every single method a transaction if you wanted to (if you were Juval Lowy/mad man), or mark entry and exit of a specific instance call of a method using a GUID as colleague had mentioned would be nice for logging.
As this is weaved in to the IL its should have not have any more a negative performance impact on calling the method, over over putting the code in to the method to do exactly the same. You do however have a whole load more flexibility to pull out and put in what you want and where (ie certain assemblies, classes or methods).
I am loving PostSharp :)