Heh, recognized reverse polish notation right away. One of the companies I interviewed at last year had me program an RPN calculator fed by CSV spreadsheets. Weirdest thing I've made to date by a pretty wide margin.
Reverse Polish notation's even cooler property is that, in addition to the unambiguity, it can easily be evaluated from left to right with a stack: If you see a operand, push it onto the stack. If you see an operator, pop two operands off the stack, compute, and push the result onto the stack. By the end you'll have a one-element stack with your result.
Really what we're seeing here, though, is a language without infix operators: all functions are of the form "<name> <operand1> <operand2>…", but grouping still works as expected.
Thanks matchu for the additional info on RPN. I didn't know about the stack evaluation benefit, that is very cool.