According to the Wall Street Journal, Apple could announce a "print subscription" offering as early as the next month or two. The publication cites "people familiar with that matter", though one of these people said it may come early next year, alongside the next version of the iPad. That sounds more like Apple's style to me.
Lots of people (and publications) seem to be under the impression that the iPad is just what the print industry needs to offset declines in revenue. Sorry, I'm still not convinced. That's not to say they won't sell some tablet issues, but the fundamental problem is still there. People don't want to pay for content, and as long as they can still access the web (from the same device no less), people are going to find plenty of free content to suffice.
I'd go so far as to say, a lot of people will find the tablet versions of publications less appealing, particularly if prices remain comparable to print versions (which so far, they pretty much have). Why not just buy the print version if you're going to fork out $5 for the issue?
Granted, app-based issues can do more. They can be more interactive and provide video and rich content (and ads). Still, users can get such experiences from the web one way or another.
Another potential obstacle is how this will work across platforms. Apple's rumored store would be one thing, but publications are going to have to create versions that are compatible with Android and other platforms for every issue - probably a great deal more time consuming and resource-using than a single app, at least if enough is going to be put into these issues to make them stand out from the print versions (i.e. that rich content).
The biggest obstacle is that few people in the grand scheme of things own an iPad. Yes, they've sold well, but it's not like the cell phone. Not everyone in the room has one. The apps will never sell like the print versions for this very reason. You don't need any device to pick up a magazine or newspaper and read it.
Tablet use will grow greatly. There's no doubt about that, but as phones continue to get smarter, I don't know that tablets will ever reach the cell phone or PC level of usage.
Now, I can see giving complimentary access to iPad content for subscribers of the print version. Or vice versa. Probably still not the answer, but it would be a nice touch (some already offer the app access to subscribers).