Well, 2050's as good a place as any to start, and that's probably about the time (give or take a few decades) that many of us can expect to live to, given a natural lifetime.
If any of you would like to continue this into the more distant future...say, 50,000 years...you may want to spill some e-ink at this site. It's the KEO Project, which essentially is a mega-blog that will be put into a satellite and launched into a long-term trajectory, to return to Earth in approximately 50 millenia. Yowza.
My personal feelings about the KEO Project are that it will be lucky to come to fruition, luckier still to survive 50K years in space, even luckier to be recovered. Its launch date has already been pushed back several years, giving the impression that it may turn into vaporware. The satellite is a passive object with no energy usage, so how our descendants will even know it's up there, or that it has returned, or where it will land (70% chance it'll hit the ocean), is anyone's guess. Still, it's a very interesting idea, and a good twist on the ol' time-capsule concept.
So what would you say to the people of 50,000 years into the future? Imagine how far we've come in the past 50 millenia and you might get an idea how utterly alien the world will be then. In 50,000 years it is virtually inconceivable that any nation now in existence would still exist then. Ditto for religions and languages. In 50,000 years no one will have any idea who George Bush, Jesus Christ, or any other name known to us now is. In 50,000 years humanity will reach unimaginable heights and suffer innumerable cataclysms. Civilizations will rise and fall multiple times. Perhaps in that time (I'm hoping much, much sooner--as in my lifetime) humans will learn to overcome our biological limits, our petty squabbles, and our Earthbound status.
I think in 50,000 years the human race may be utterly alien to us now, physically, culturally, linguistically, technologically, etc. But I am optimistic that in one form or another, we'll still be around. Here's hoping.
P.S. Thanks GP for the invite!