It was started as a once-off thing by Phil Wyman and John Smulo in December 2006, when they proposed that a group of Christian bloggers, mostly with an interest in the "emerging church" movement, all blog on the same general topic on the same day, to compare and contrast different views. The topic of the first one was Syncretism.
Some people liked it, and agreed to continue to do a synchroblog once a month. This month's topic, published today, is "maturity" -- what it means to be adult. You can find my contribution at Adult content, with links to the other contributions -- there are nearly 20 contributions this month.
There is a mailing list for the regular participants in the synchroblog, where they discuss future topics, when they will post etc.
The idea seems to be catching on now.
There is at least one other Christian synchroblog, and next month they will be having a post-charismatic synchroblog. As Jonathan Stegall puts it
If you're interested in that one, you can find more at robbymac: Getting Here From There (Synchro-blog Invitation)
Should you be unaware of what a post-charismatic is, give the above blogs, as well as Mike Morrell’s, parts of this blog, and several others, a perusal. In any case, I had actually been planning to give an account of my first encounter with the Spirit. In the near future, I believe I will be telling my story, in a very broad way, for the people of Revolution.
There has also been a Pagan/Mythology Synchroblog, and several more. There's even a controversy about synchroblogs. To see just how popular it is becoming, check on Technorati for synchroblog. You might be surprised.