Sensi,

I bow before you.

Like You (or a proto-You), I'm a web developer. While I don't have as many macs as your throw-down posits, I do have two. Outside of Subversion, I can't conceive of the Grand Unified Sync Theory that can reliably and automatically manage many machines unless you're willing to designate one as a master and all others as identical twin children. If the parent-identical-twin-child model would work for the purposes of your GUST, then my solution would scale. Another assumption is that all machines in this model are owned by the same person. This model definitely wouldn't work if different people had to stay in sync (in which, again, Subversion is the only viable solution I know of).

Enough preamble...

My main dev machine is a Mac Pro desktop, and I have a Mac Book that I use to take things on the road on occasion (okay...take things to my living room on occasion). The Mac Pro for most things is designated the Parent, and the laptop is the child, meaning that by default I want to push changes from my Mac Pro to my laptop, and if changes are made while I'm on the road, I need the ability to push changes back.

Mail: I use imap in Mail.app to keep my two-dozen or so mail addresses contents in sync. To set this up initially, I had to copy the contents of Mail's library folder from my main machine to my laptop to preserve folder structures, rules, and preferences. Once done, this needs to be redone only when changes are made or rules are added. For this, I use a non-scheduled ChronoSync. This would scale in any situation and is a nearly zero-maintenance option that satisfies your "on an airplane" requirement as well as any solution can.

Keychains: I use 1passwd which is a lifesaver. Simply using ChronoSync to copy my keychain works just fine.

iCal: I use Spanning Sync. This allows my wife to control--er, contribute to--the calendar from her non-Mac office via gmail. I run into the occasional sync conflict, but this is expected and manageable. ChronoSync is pretty much all-or-nothing in unattended mode, and it blew away too many changes for it to be an viable solution to this problem.

Development Content: For my web sites, I use Subversion version control. After all, developing on multiple machines was what version control was made for. Unlike mail, cal, or to-dos, my dev folders don't need to be identical. They can live independently of one another, and SVN takes care of the details.

Less important stuff: I use scheduled ChronoSync jobs to manage all other day-to-day file syncing. I awaken my machines at 1am (System Prefs > Energy Saver > Schedule...) to run automated backup scripts (outside the scope of this post). While ChronoSync can't be scripted via Automator or cron jobs, you can schedule jobs internally within ChronoSync to kick things off when you know the machine is going to be on. This works well except for those times I forget to leave the lid of my laptop open. Since I'm not the only person in my house that uses the laptop, this happens often. Fortunately, it's easy to kick off the otherwise-automated ChronoSync script and it only takes a minute.

This combination proves to be an effective--if not convoluted--solution that lets me grab my laptop and go with confidence that I'm in sync.

In my heart of hearts, I still long for your Grand Unified Sync Theory solution. As your thread proves, this is clearly an itch that needs to be scratched.