Before OmniFocus 1.0 was released, there was talk of Omni Group building a web interface to the app. That feature was shelved early on but private users picked it up and continued (maybe it even became open source -- not sure). You can probably find more information on it in the
OmniFocus Extras forum.
And recently, hints of a new web interface developed by Omni Group have
surfaced again.
But until a web app version becomes available (if it ever does), your only real options are to either get an extra Mac or an iPhone/iPod touch.
-Dennis