Interstellar probes probably won't happen, unless they can "piggyback" off of some other essential technological development. Fortunately, such a development is indeed feasible.

At some point in the relatively near future (ie the next time there's a Tunguska-sized impact or thereabouts), humanity should realize the need to build an earth-protection system. The best way to do this would be with a large photovoltaic farm at a lagrange point, (probably constructed from lunar materials), with a VERY large laser attached to it. When an impactor that threatens earth is detected, the laser could be pointed at it, thus gradually pushing it into a less harmful trajectory. At other times, the laser could be put to other uses -- eg sending optical signals to distant star systems which possibly contain intelligent civilizations, or pushing laser-sail vehicles around the solar system -- or out of the solar system entirely. A very lightweight STARWISP-style vehicle, weighing only a few ounces, could be accelerated to a fair fraction of the speed of light, over the course of several weeks. Since we need to have the laser *anyways*, doing this would be relatively cheap, and we could have our first probe arriving in another solar system within 25 years of its launch.

I'd wager that such a laser will probably be built around 2050, and that an interstellar probe will probably be sent out within the following decade.

As for sending *humans* to another star system... I don't think that'll happen for quite a while longer. Say, 2200-2300. But when it does eventually happen, it won't be any big deal -- it'll just be some isolated and self-sufficient Kuiper-belt colony, quietly deciding to light a fusion drive and cut loose from the rest of the solar system. They'll drift in interstellar space for many, many generations before arriving anywhere else -- but by the year 3000 or so, I expect that there will be humans living around a handful of different suns.

Cheers,