I think this forum should have a sticky for version changelogs (official, and what's been found through source-combing)... In case this ends up being the changelog thread, I don't mind updating it as more info's released, here we go.

2.0.3 (327610024):
http://www.mobileread.com/forums/showpost.php?p=469939&postcount=10
unrequited, I've confirmed that 2.0.3 recognizes the new TTS-disable flag in books sold on Amazon's site that are TTS-disabled. TTS blocking does not work on 2.0.2 or earlier.

http://www.kindleboards.com/index.php/topic,7303.0/
Since I updated my Kindle 2 with the 2.0.3 patch, I tried it again and it seems much much faster.
Wow!! Much faster!
note: others like myself have noticed no speed increases, but it may be on a website-by-website basis

http://www.mobileread.com/forums/showthread.php?t=45528
Last night I upgraded my K2 to 2.0.3, and now I can't turn on USB networking. I get to debug mode just fine (typing `help shows all the commands, including `usbNetwork and `usbQa), however typing those seems to do nothing.

2.0.2 (309510017):
http://www.mobileread.com/forums/showthread.php?p=397296#post397296

INSTALLED AND TESTED
Works w/ Kindlefixed files.
Also installed the Kindle2 screensaver hack ( http://www.mobileread.com/forums/showthread.php?p=380345#post380345 ) and it worked also.

NOTE: make sure to backup your documents folder (and any other necessary data you may have) before making the update. If I hadn't made the backup, I may have lost data and had to find it all again. Kindle 2.0.2 update would NOT install until I ran "Update_kindle2_restore_default_screen_savers. bin" and after I ran it, the USB connectivity would not work correctly. "I Reset to Factory Defaults" to get it back to a working 2.0.1 and then updated to 2.0.2
note: a lot of other people are just unhacking, updating, and hacking again without issue, but backups are always a good idea whenever performing an update.

http://www.kindleboards.com/index.php/topic,5564.msg116077.html#msg116077
Just updated, and the UI definitely is faster. Page refreshes are faster. I can confirm at least one bugfix as well. When reading manga/comics (or viewing images in the pictures folder) "Partial Refresh" finally works. Whenever you turn the page, it blanks the screen, refreshes the top half of the page first, and then refreshes the bottom half of the screen so if you're reading comics/manga you can change pages even quicker.

http://www.mobileread.com/forums/showthread.php?p=397814#post397814
Looking at the published gnu linux kernel sources provided by Amazon, only 4 files are different:

arch/arm/mach-mx3/dvfs_v2.c
arch/arm/plat-mxc/isp1504xc.c
arch/arm/plat-mxc/wdog.c
sound/arm/mxc-alsa-pmic.c

These are the kernel (2.6.22) mods from the last Kindle version, 2.0.1 (303870012).

Until Amazon posts release notes, we won't really know what was different in the functioning of the Kindle software, but the kernel changes look like adding in a counter to reboot (6 times) and setting the minimal soundchannels to 2.

2.0.1 (303870012):
http://www.mobileread.com/forums/member.php?u=1182
- the power management routines of the i.MX31 CPU and MC13783 PMIC
- the battery charger routines
- the broadsheet watchdog timer
- the eInk frame buffer device

v2.0 (291330095):
Initial release

reserved for extra room 1

reserved for extra room 2

reserved for extra room 3

I'm not sure uploading Amazon's files here is a good idea...

I'm not sure uploading Amazon's files here is a good idea...

I took them down. Forum policy.

2.0.3 added, notes to follow from user discoveries in the coming days

udpated 2.0.3

updated

unrequited, I've confirmed that 2.0.3 recognizes the new TTS-disable flag in books sold on Amazon's site that are TTS-disabled. TTS blocking does not work on 2.0.2 or earlier.

thanks! updated

Hi!

Amazon support ( http://www.amazon.com/gp/help/customer/display.html?nodeId=200324680 ) still links to http://www.amazon.com/update_kindle2.bin but the file is not available anymore..
unrequited do you have the file and can I send you a PM ?

I just clicked on it and it worked... that's Kindle 2 v2.0.3 linked here: http://www.amazon.com/update_kindle2.bin

weird.

Here it says:
We're sorry, this title is not currently available for download.

We are investigating the difficulty with this title and ask that you try again in 24 hours. If after that time you are still unable to access this title, please contact us for further assistance. We apologize for the inconvenience.
Thank you for shopping at Amazon.com!

Weird..
That's maybe because my new Kindle isn't registred yet. I live outside U.S, but I'm going to try anyway.

That did the trick, thanks for your fast answer unrequited!

So any update on 2.0.4?