In the blog post about the release of Windows 10 Preview Build 9879, Microsoft announced that they will be releasing ISOs for the Windows 10 Preview Builds in the future.
Getting the update & install data from our Preview Builds mechanism is super important for us. It helps us ensure smooth ESD distribution, download, and upgrade success for this program going forward, and also will help us ensure great upgrades for people once we release Windows 10. So we’re going to release the ISOs at the same time as we publish to the Slow ring. That means if you want to be FIRST and FASTEST to get the build, you’ll need to use our Preview Builds mechanisms (either automatic or Check Now in PC Settings to download.)
But for the Windows 10 Consumer Preview build coming in January, Microsoft will be releasing Windows 10 Consumer Preview both via Windows Update and ISOs at the same time during launch.
@NaThanPhamIT Next build we release will include ISOs as well as published via Windows Update to upgrade existing installs.
— Gabriel Aul (@GabeAul) January 5, 2015