I will be surprised if we don't see electronic versions of products from WotC's back-catalog become available, or start to become available, sometime in 2013.
There seems to have been a sharp increase in the number of back-catalog novels that WotC is releasing each month, in various ebook formats. So there are certainly folks at WotC focused on electronic distribution. Strategically, 2013 is probably also a good year to re-launch electronic sales of out-of-print items. Most of the announced print products for the year are reprints of old material, sales of which are likely to be boosted by easy availability of compatible adventures and accessories in electronic form.
If availability of electronic RPG products has to be linked to DDI subscriptions, I'd like to see a model where they add 5-10 new PDFs to their library each month, and being a subscriber lets you download anything currently in the library. That's how things currently works for Dragon/Dungeon content. If you pay for a one month subscription, you can download everything released up to that point, and keep your own copy of that content, if you really want to.
Of course, that model would mean that not everything out-of-print immediately becomes available again electronically, but I'm fine with that, as long as when new PDFs are added to the library each month, they are higher quality PDFs than those previously sold. There are enough products for BECMI, 1st, 2nd and 3rd edition, that releasing one or two new PDFs for each edition each month could persuade fans of any edition to see DDI as worthwhile subscription -- at least for the 2-3 years it would take to finish releasing the entire back-catalog.