I think given current internet world and the direction most of the software giants are taking, it is a good idea to take the time and study further into distributed systems. I agree that you learn a lot on the job but there is something to be said about the importance of having sound fundamentals. I went through a similar phase and ended up going for a MS in distributed systems and have never regretted that decision. It makes you think more concretely and exposes you to things that make you think of scalability, complexity, high performance, high availability and other aspects that are the essentials of any complex (more importantly - any system of importance) that we think of.

So, bottom line, go for a Masters but choose your subjects/areas of concentration wisely and remember that you are there for digging deeper not wider.