With the introduction of the Kindle 2 and the Kindle for iPhone application came Whispersync: the ability to sync the current place in your book between devices. Amazon also introduced text-to-speech capability, allowing you to listen to your books aloud while still maintaining said place within your book. This, gentle reader, is but a temporary method to integrate text and audio.
Remember Amazon's acquisition of Audible.com?
Once Audible is fully-integrated with the Amazon Digital Services platform, I'll bet that Kindlers will be given the ability to read and listen to the same book and never have to fumble for the place where they left off.
Of course, publishers are not going to just give away two copies of the same work for which they normally charge full-price, but I envision some sort of bundle where the combined cost of the Kindle text and the Audible audio will be sold at a 25% - 50% discount vs. purchasing each of them separately.
Of course, device support will likely be limited to those that are networked and allow third-party applications. At the moment, that means the iPhone/iPod Touch, BlackBerry, Android, Palm Treo, Palm Pre, Windows Mobile, etc. although it's certainly possible to accomplish this feat with a desktop application acting as an intermediary. Audible has done this for years - when you sync your portable media device to your desktop, it notes your playback position, allowing you to listen where you left off using your desktop media player.
When might we see this?
That's unclear, but it will begin the day Audible customers receive an email that says something like, "Dear Audible Customer, you'll now be getting your audiobooks through Amazon.com..."
I'd say, look for it within a year to eighteen months.