Go to Amazon and order one of their discounted MobileMe boxes for a bit more than a dollar a week. Now you've solved your webdav supplier issue for a year, and configuration is not really any harder than it is for the Bonjour method. Over the course of the year, you can fool around with seeing if you like one of the free services mentioned elsewhere (there's a thread discussing them on the main sync forum, I think), or even set up your own if you want to get your hands dirty.
Or, instead of Amazon, go directly to
http://www.me.com and sign up for a 60 day free trial, and do the above process 6x as fast :)
One thing: don't turn on the Mac's "iDisk disk sync" (shouldn't be an issue, you are using Dropbox) unless you tell OmniFocus to store its database in the Backup folder instead of the Documents folder on the iDisk.
Dropbox doesn't do webdav, and 3rd party iPhone apps can't get at each other's data. Therefore, OmniFocus on the iPhone isn't likely to be able to use Dropbox as a sync mechanism directly. As for having two different sync mechanisms active, that would require clients to serve as gateways, and while it isn't an impossible bit of coding, there are a lot of other features that get asked for on the forum much more frequently that we've been promised will be delivered but have yet to arrive, and no apparent champion of the idea at Omni, which is my long-winded way of suggesting that maybe you shouldn't hold your breath while waiting :)