With kernel mode-setting drivers the screen already flickers minimally when booting, but Keith and Steve were seeing whether they could manage a truly flicker-free boot process. In particular, if the initial mode-set when loading the i915 DRM could be avoided.
With Ubuntu (and some other Linux distributions too) using GRUB2, it gets the boot monitor in a "reasonable configuration" using VBE calls, which sometimes works out well but the mileage can vary depending upon the BIOS and whether it knows about all available modes. This process of finding the proper native mode might be improved with UEFI-based systems. In this mailing list message to open-source Intel developers, Keith goes on to explain at length the process then of continuing a flicker-free boot process and all of the changes that must be made.
In the end, for those not caring about all of the technical details, a flicker-free boot for the Intel DRM/KMS driver is still a work in progress. However, Keith does have some preliminary patches available that come as a result of his weekend hacking. Hopefully at some point in the near future they'll be ready for integration.