Another vote for Python due to how easy it is to learn, and how powerful it is once you do... but have a look at Sage - its browser-based python for mathematicians. Sage has all the scientific and numeric libraries running by default, works in any browser, uses Python as its base language and is used to teach statistics and calculus in some universities because it makes things so easy.
So to split my answer: learn Python, but use it in Sage. Its free, open source and there are sage servers you can use all over the place for free - and because they are web (or cloud) based, your work can be accessed from wherever you are
Hope that helps : )
PS: Shadowman, I can program in just about every language you listed in your pro-Java rant... and I teach programming. From personal experience I can tell you that teaching someone Python is considerably easier than teaching someone Java - and Python is every bit as powerful. The underlying concepts are the same though. And Python as I have described is going to be much more useful to a mathematician than Java will be. It wasn't a misguided flame against Java by someone who doesn't know it - it was simple opinion, as are all answers here. Like it or not, thats what you get when you ask a question - opinions.
Oh - and you bit about Java being more powerful and cross-platform... Python is powerful and cross platform, so you really aren't making a great argument there...