i will be using this definition of "civilization":
advanced development of society: an advanced level of development in society that is marked by complex social and political organization, and material, scientific, and artistic progress.
[Definition courtesy of Encarta.com]

a funny thought is that no ancient civilization survived into the modern day. once they got too big, every civilization collapsed under its own weight or under the foot of a larger civilization. it is also easy to see that history repeats itself over and over and over and over again. so what are the chances that our modern civilizations will last indefinitely?

so the interesting bit is to wonder in what manner will our civilization reach its demise. will we become too unmanageable and factionalize? will better managed "barbarians" storm in to take advantage of a bloated bureaucracy and take over? or perhaps technology will allow governments to control its citizens to the point where they cannot rebel or factionalize, thus keeping control? better technology is an advantage they have over the ancient civilizations. but can they really stay invincible? won't they eventually be overtaken? will our civilization's demise be sudden and forceful or will it be subtle and predictable? could looking at the demise of previous civilizations aid in that prediction?

will the world eventually become a united world ruled under essentially one government? if that global government falls will the entire world be plunged into a dark age? when everything becomes digitized and all advanced knowledge is kept on computers, a collapse of society and its infrastructure would easily wipe out our access to that advanced knowledge. it will then only take a couple generations for us to lose all advanced knowledge.