The port is now officially available, builds can be downloaded from the official (unstable/development) autobuild.
Also see http://chdk.wikia.com/wiki/Downloads

Cameras running older firmware versions need to be upgraded to the official 1.0.2.0 (aka 102b) firmware. Even though the firmware is the same, each model needs its own firmware upgrade file: sx270, sx280

Limitations:

Some frame buffer related functions have issues (edge overlay, zebra)
CHDK overlay (OSD, menu) draws slowly. ~~The default font is tiny,~~ bigger RBF fonts are selectable for the menu.
Certain Canon graphic elements make the whole CHDK overlay blink. For example: blinking battery icon, scrolling images in playback mode.
Rebooting from script is not working.

Heavily modified, outdated original text follows.
[hr]
First test release for the 102b firmware revision (this is the 1.0.2.0 official Canon firmware).
It has a simple version check, if that fails, the camera will go into infinite loop and the battery has to be removed.

Warning: as a test release, it might be unstable.
Known problems:
- ~~CHDK display is not erased properly. To clear the screen, enter and exit the Canon menu or use the "control dial"/jogdial in playback mode to step to another picture. This is annoying, I know.~~ Prealpha2 uses a newer method which is also annoying, but differently.
- ~~The jogdial has no CHDK function. The reason is that there is no jogdial task in firmware, a new control method will have to be found.~~ Fixed in prealpha2.
- No frame buffer related CHDK functions work - the pixel format is different, support for that will have to be added. The (probably incomplete) list of unavailable functions: histogram, zebra, edge overlay, auto iso, motion detection, PTP live view.
- ~~The camera will always start in playback mode.~~ Prealpha3 can now start in rec mode.
- ~~Some colors will look differently than intended, the previously black 0xff color is now pink.~~ Most color issues are fixed in prealpha3 (except the non-existent transparent color and color bleeding caused by YUV).
- Most modules are not aware of the bigger resolution.
- The default text size on OSD is tiny but readable. The OSD editor can be used to increase the size of OSD elements, the menu can be made better with loadable fonts.
- Drawing of the CHDK overlay is slow.
- ~~'F Number', 'Exposure Time' is bad in DNGs (in exiftool output)~~ Fixed by using native modules since prealpha3.
- ubtest reports
'uBASIC:206 Unk key'
after 'done', file is OK... fixed in prealpha10
- Movie mode and remote shooting (filewritetask) is not supported at this stage.
- ~~DNG files have all their black borders.~~ Borders corrected in prealpha3.
- Messing with the CHDK ND filter control may crash the camera - all cameras with unofficial ND have this problem.
- ~~Subject distance override is completely untested.~~
- ~~The CHDK GUI is unusable (distorted, falls apart) when using the analog video out connector.~~ Fixed in prealpha3.
- Overrides are not effective in high speed burst mode.
- ~~Edge overlay re-enables itself randomly (the module is not included because it wouldn't work)~~
- The reboot() script command is not working.

The benchmark is working, please post your card speed results if you have fast UHS cards.
Note that the test release only supports FAT32 or FAT16 cards.

To load this test release, you need to make the SD card bootable.

Source is here, this toolchain can be used to build it. ~~Modules are not currently built, the included modules were compiled for "regular" CHDK (with the gcc library routines included - changeset 3241 reversed).~~ Modules now build correctly. Compilation was tested with the 32-bit version of the above toolchain (Linux and Win32). Newer source repo is here (changelog).

For generic DIGIC 6 related development talk, this thread is probably more suitable.

edit:
tv-out issue added
edit2:
more issues added
edit3:
more issues, link to newer release
edit4:
another release, issues changed

edit5:
first release removed (I don't think the 67 is the real download count, some people have trouble downloading attachments and re-try many times).
chdk_sx280_sx275_sx270_102b_test1.zip (656.69 kB - downloaded 67 times.)

edit7:
prealpha5
edit9:
more issues
final edit:
official release
another final edit:
removed some comments that no longer apply