Intrusiveness does not equal effectiveness. Some people may have genuine reasons not to enter the data or to delay it, for instance, they know the address would change or are accomplishing the same task by other means.
I think a prominent notification bar on top (like GMail does) is almost always preferable to a modal pop-up.
- User needs to perform a task, perhaps an urgent one. Modal dialog impedes their flow and distracts them from their main task.
- Once the user dismisses the modal dialog (which they are likely to do), they are not likely to come back and do the nag task (they'd have: to remember it, want to do it, and have a way of finding it in the UI). Their next chance to do the nag-task is when popup opens again (at the wrong moment).
- The notification bar, on the other hand, is always present, so they can easily launch the nag-task at a more convenient time (when done with the main task or when waiting for something to complete).
- The only disadvantage I see is that notification bar can be more easily ignored if it does not sufficiently contrast with the rest of the page. However, this is offset by the fact that it is on top of the app, taking prime visual space until dismissed (rather than showing up for a limited time at a possibly bad moment).
When deadline comes around, you may have to force the users to complete the nag-task, with no way to postpone it. Before that time, I think a modal popup would only serve to annoy them.