I just finished the data science coursera class, and while we used Python, we didn't get into R. I've played around a little R on my own, and while I certainly don't think it's too difficult to learn for a programmer with a math background, as a programmer I feel much more at home with Python. Given the choice, I'd rather use Python than R just because it feels more natural to me.
If Julia is a truly a programming language, I'd agree that it would be more of a competitor with python than R (in the sense that its audience would be people like me who would lean toward python)... but I think it could be very successful by making it very easy for a programmer to stay within a programming language. In other words, it wouldn't compete with R, it would compete with python by bringing what you get from R to a programming language.