The news comes from Steve Troughton-Smith who has been combing through the code in iOS 8 beta, and he has apparently found references that point to split-screen multitasking functionality. He says that the Springboard (aka the iOS homescreen launcher) has code that refers to new "main-screen-canvas-sizes", which would allow two apps to run side-by-side on screen. The code already allows for each app to take up half the screen, or one to use 1/4 of the display while the other uses 3/4. Additionally, when using the multitasking switcher, a screen that had two side-by-side apps would appear as one item in the switcher.
Troughton-Smith goes on into more detail about the specific sizes for each option: 1/4 screen being about 341pt, 1/2 screen as 512pt, 3/4 screen as about 682pt in width, and obviously 1024pt as fullscreen. He even posits that the apps would be resizable, but it seems like they would only be resizable to those preset dimensions.
We'll just have to wait and see if Apple gets this going in time for the full iOS 8 release, but it's hard to imagine that it wouldn't be announced at the developers conference, given that developers would likely have to update apps to fit the new sizes.