Towards the end of HP6 (Half Blood Prince), we see Dumbledore atop the Astronomy Tower, together with Harry. His condition was rather pitiable from drinking the potion, and he asks Harry to call Snape.
Now, why didn't Dumbledore simply use the "patronus" method to call Snape? (We have seen that the Order members sometimes communicate via their patronuses). It would have been much simpler- and there was no guarantee that Potter was going to call Snape. (Harry himself wanted to call Madam Pomfrey).
One possible reason could be that drinking the potion deranged Dumbledore. However, it's stated in the book that the sign of the Death Mark on the top of the Astronomy Tower steeled Dumbledore. He didn't lose his ability to do magic-he undid the protections around Hogwarts, so that they could reach the Astronomy Tower on their broomsticks, and he also put a body-bind curse on Harry.
So, if he didn't lose his power to do magic, or the power to think (his conversations with Draco and other death eaters later show that Dumbledore had a clear head towards the end too), then why did he not use the tried-andtested method of communicating with Snape via his Patronus?