Is it true that every possible finite number sequence is present in the decimal expansion of pi?
I don't think there's any way to prove that. It's a completely determined sequence to the nth digit. I think you can only talk about the characteristics of what actually has been calculated.
For a truly random infinite sequence, you could say that any finite subsequence of digits has a non-zero probability of occurring. For a uniform distribution (each digit 0-9 has the same probability of occurring), the probability of any given sequence of length k occurring is [tex](0.1)^{k}[/tex].