You could simply use the interface class. Whatever class contains the logic for creating an instance of A will contain two arrows to the time-line of the interface class. The first line will indicate the instantiation and the second line will indicate the call to method M().
If it is
important to detail both sequences of when the condition is true and when it is false, you would create two sequence diagrams (one for each case). In these diagrams you would use the actual concrete class instead of the interface class. You could add a stereotype to the class indicating that it is an instance of A with <<A>> (a nice touch).
Note, you have to determine whether this detail is important enough to include on your diagrams. Personally, I don't think so.
Also, the way you are creating the objects should be looked at again. In your example, you have a significant syntax-level dependency upon the class name.
Very Bad! I suggest you read up on the Creational object-oriented design
patterns and rethink how you create objects.
Good luck!
[ July 08, 2008: Message edited by: James Clark ]