a couple words of caution for those working on ps3 flash.
first of all, writing to flash is possible with couple tools, personally i use dev_blind that allows ftp access with read/write (root) to flash0.
i prefer testing this way over the loader because of the nature of the tool.. when something freezes or doesn't work, is it the theme or the app running files from wrong places ?

secondly, it's very much possible to brick the flash so that you can't recover. the recovery mode is a separate prx in the flash, and with it, it has an RCO associated with it.
if you destroy this file the recovery menu will not work as an incorrect rco will make the menu freeze upon load. I did find it but i forgot what rco it was. I'll post that here once i run into it again.

it's nice to see interest spawn in this area.
i thought im all alone :P

there are decypters for everything..
they're called fail0verflow tools in general, you can rip open anything encrypted with sony's kirk key.
this involves all executables and files inside firmware files, along with the firmware file itself. (if you're gonna do this, the split .tar for dev_flash inside the firmware .pup is an donkey to extract due to the nature of the tools)

decrypting the firmware is relatively pointless though, as the same files can be FTP'd out of flash0 with dev_blind

what comes to that recovery, while it does work it's originally designed to be used for downgrades and it messes with the system very badly. incase it goes wrong it has a chance to bring your ps3 to the state it doesn't power on anymore. use only as a very last resort.

messing with actual dev_dlash gives more reliable results when it comes to RCO testing.

the flash, while gigantic compared to psp's still has it's restrictions, and there's also size limits of elements like on psp that are being ignored by this.
it's possible to generate files that work on loader and get glitched when written to flash.

it's also good to remember that there are 2 flash sizes, 128 and 256.
assuming that ps3 xmb mods ever reach the levels of psp's, this is bound to cause issues, i haven't done too much testing as of yet and I've already once generated an rco that was simply too massive for the ps3 to handle, the graphics would simply not load, and in addition to that, creating custom add-on's by adding files to rco's and more parameters to xml's the files get very big very fast.

as much as i'd like to use it for testing and save me from bricking a second ps3, i just don't trust it to give accurate results.
The last thing i want to do is create and share a theme and have a hundred people brick a ps3 because it doesn't work correctly :P

well well...

after a week of bugtesting and endless reflashing and a couple of brick recoverys (lol)
here wee go:

   

V0.3 of my little clock mod for 3.55-kmeaw
* finally compatible with both slim and fat models
* no more corrupted graphics in ingame XMB

it's a step forward but there's still things to do.
my research led me to see that fat and slim flash files somehow behave very differently.
i used modules from a fat one on a slim and it resulted in imminent freezing and some random gui slowdonws. Not a good sign.

i used modules from 3.55-kmeaw installed on a slim and managed to produce a stable set of modules that works on both fat and slim now. I also got rid of the clock face corrupting on imgame xmb which bugged me for a long, long time.

im not going to post any actual research results until i really know how things work, but
it seems that the first steps towards proper flash0 themes have been taken :)

*MOD EDIT:  I had to put spoiler tags on the image, as it is so huge that it takes a exceptionally long time o load up...   :/         ~shizzy

if you mean the main icons (tools, users, music, video) etc, they don't seem to be inside any RCO and I've yet to find where they are.
there is an RCO file with these icons inside (forgot which one it was) but changing htem only changes the icons for INGAME xmb, not the normal one.

there's several possibilities...

1) they are inside an SPRX file (i looked at few of them but havent found graphics inside of any so far)
2) they are in VSH(main).rco which is not present on the flash0 that is visible to us.

i have found references to a pile of rco and sprx files that are not present in the flash0 from the bootloader and I've yet to figure out where they are.
..however, i also found a string from the bootloader that calls a partition called dev_SYSTEM which no one seems to know what it is....

there's still a lot of work to do before proper 100% flash0 themes can be done :/