So I have two Kindle DX's ATM at software version 2.5.2, because i haven't gotten around to sending my DX with the broken joystick back quite yet. So I left it registered to my account.
Checked it today, awesome, update available! I don't know what 2.5.3 does, but if you want it, since 2.5.3 isn't on the manual download page @ Amazon, this is the thread to be at.
The funny thing is, it was only available when I plugged in my Kindle to by Computer via USB with the wireless on, the first time I tried to grab the update, the wireless was off and it didn't show up in the root directory. But I was certain it had downloaded, I had the option to select Update Your Kindle and everything...
Anyways, here's the update link, again this is for the US Kindle DX (Sprint) and it will only work if you have that Kindle, and only if you already have Kindle update 2.5.2:
http://www.mediafire.com/?zlnufzmyzwi
Firmware. 2.5.3. Kindle DX US. Sprint.