What does "Not fully compatible with OmniFocus for Mac 1.6" mean?
 

Since OmniFocus 1.7 for Mac is fully compatible with OmniFocus 1.5 for iPhone, but not with OmniFocus 1.6 for Mac how should I upgrade since I sync between two Macs and cannot upgrade them simultaneously?

Sorry, that was a bit confusingly worded - if the Macs are running OmniFocus 1.6.1, there's no problem.

Once version 1.7 syncs with your server, 1.6.0 or any previous build of the application just won't be able to understand what all the perspective information is doing in the database. That's what "not fully compatible" means.

If you're worried, you can grab OmniFocus 1.6.1 from [URL="http://www.omnigroup.com/ftp/pub/software/Archive/MacOSX/10.4/OmniFocus-1.6.1.dmg"]this link[/URL].

How will this work if I don't upgrade one of my computers for a couple weeks because I am traveling? When I return to it, will it corrupt the synced database, pop up an error message, just show a stale display until I upgrade it to OF 1.7, or something else entirely?

OmniFocus 1.6.0 and earlier will not be able to read the sync database in the new format and may crash while trying to do so. They won't damage any of your information, though. I've been opening all four versions (1.6, 1.6.1, 1.7, and now 1.7.1) in various combinations over the last couple of weeks; no damage yet.