[Brian]

Apologies for the confusion! This is what I get for leaving my Mountain Lion-upgraded laptop at home yesterday and posting without empirical testing. ;-)

I followed up on this with a couple more folks today: here's what appears to be going on. It does look like Calendar will let you create local calendars, but it’s important to remember that to-dos are now shown in Reminders, which defaults to putting new lists in iCloud.

Aside from amelchi's experience, we've gotten at least one additional report of the feature continuing to work after installing Mountain Lion. Folks with this already set up may be able to avoid adjusting their workflow for the time being.

The fact remains that this release of the OS officially moves the programming interface this feature depends on to "deprecated" status. This is basically Apple saying "this works for now, but we may break it or remove it if we need to at any point from here on out". Even if there turns out to be a way of making a new “on my Mac” to-do list, the feature will stop working when Apple removes the API it depends on.

Upshot: people with this already set up may be able to use the feature for a while longer on Mountain Lion, but it's virtually guaranteed to break at some point in the future. To reduce the potential for greater confusion when that happens, the 1.10.3 release will remove the UI for this feature when running on Mountain Lion. We don't want additional customers building their workflows around this.

We apologize for the inconvenience this causes folks who were able to put the feature to other uses, but it has far outlived the intended purpose, which was syncing to-dos with pre-iPhone PDAs.

Rather than spend more energy keeping this feature limping along, we’re going to continue working on giving OmniFocus a new, more modern interface and foundation that will benefit all our Mac customers. Thanks for understanding.