QUOTE
"A jack of all trades is a master in none"
You know one hears this quite often and it is often a quite true statement but on the other hand it is really not the best bit of logic.
Innovation happens because of diversity. Becoming a "Master" of a particular language or technology is really rather useless. I would rather work with people who have dabbled in many things that someone who just knows one or two.
The truth is, mastering any particular field in the industry means that you have a general knowledge of many many areas. I routinely work in many languages, I routinely pull for experiences in other languages.
I agree that you should not try to become a "Master of all" but definitely, definitely, definitely allow your experience to wander about. Read, and browse. Don't be the guy with "25+ years experience in C++ payroll systems" who does not know what Excel is, or that Java is no longer just a toy language the guys at Sun are kicking about. I can tell you, that guy has been working as an accountant for last year at 1/2 his salary because he can't seem to get another programming job. (true story).