From one point of view the probability that "aliens" should trake the initiative within a century seems negligible, since there is no evidence they did come here the last 5 billion years(so . perhaps les than one in 50 million chance?).
The opposite situation not very likely either: humans "discover" E.T.s?

I wasn't being clear. What I meant was that we are 30-40 years from being able to discover whether or not there is a deep biosphere on Mars, Or Europa, not that there actually is one.

However, based on what I have read as an interested layman, I do think it's possible and there are many scientists who feel that there is an even chance, or are even convinced that there is still microbial life on mars.

edit : MSL may be able to tell us directly if there was life, and its due to begin operations in 2013. There are other missions beyond that. I dont think its unlikely that we would be doing science that would tell us these things in 30-40 years.

I concur.
Given how rapidly our understanding of both Mars and Luna is changing I'd say it's quite likely such things will be discovered, if there, within 30-40yrs.

__________________
"Where the telescope ends, the microscope begins. Which of the two has the greater view?" - Hugo

"Men occasionally stumble over the truth, but most pick themselves up and hurry off as if nothing had happened." - Churchill

While I very much hope you're both right (I would be absolutely thrilled),
I think in that time frame we will only be able to confirm life on either Mars o Europa if it is either really abundant or to be found exactly where we expect to find it.

If there are only small pockets of life (or it's very different from what we expect), it might 'resist' detection way longer.