Pokémon Omega Ruby & Pokémon Alpha Sapphire both launch in November, and Nintendo UK has confirmed that a special demo of the new monster-catching adventure will be made available on October 15th.
This "Special Demo Version" contains "gameplay that players won’t experience anywhere else", and will allow players to Mega Evolve a "familiar" Pokémon. This Mega-Evolved Pokémon can then be carried over to the full game in November, as well as other items. All of this suggests that the demo will be a self-contained story, rather than a portion of the main game itself.
Codes for the demo will be available in the UK from Game, Amazon, Smyth Toys Superstores, ShopTo and the Nintendo UK Official Store from October 15th all they way up to launch. We'll also be distributing codes on the site, so be sure to keep checking back.
To coincide with the launch, Nintendo has confirmed that two new versions of the 2DS will be released, which you can read about here.
While this is only confirmed for the UK at present, it would be safe to assume that Nintendo of America and Nintendo Europe are planning similar demos. We'll be sure to report on those once we have official word.