Well i have a lot of experience with bricking ION.
I have LT28i but i think it does not make any difference than the normal H version....
First off i installed paranoidandroid rom via CWM, it all went smooth and it said in instructions that you have to restart your device and then enter the fastboot mode, well after i pressed restart from CWM it started with boot loops, sony logo fades into yellow and so on... I knew that there is soft reset (press volume up + power, if you hold it longer it does hard reset)...
So i started off with soft reset what did not make any difference, still boot looping sony logo.... next step hold it a bit longer and hard reset.... after that phone dead, no led no screen nothing- tried SEUS and other software didnt make any difference... left the phone on the charger for 1,5 days, then i saw red charging led blinking but nothing (i used 2AMP charger, it did not do anything with normal charger), after couple of hours phone i saw sony logo and again boot loop, i did find a solution tho, just after hard reset (volume up+ power for 30 sec), then wait 2 seconds (before it starts to bootloop again, have to be precise) i pressed volume up and inserted my usb cable what was connected to laptop. Phone successfully entered fastboot mode, and then i was able to flash OC-ed 1.73GHz kernel into it and after restart everything beatiful, no bootloops everything fine!
Then i got sick of paranoidandroid (because i loved the 3d panorama feature in stock ics and i have 120hz 3d screen on my pc) and decided to revert back to original software. I thought it was easy, but well it was not. I had 2 different original FTF files for flashtool, but flashtool did not flash them !!!
[09/032/2012 15:32:54 - INFO - Device connected in flash mode
09/032/2012 15:32:56 - INFO - Selected LT28i_4.0.4_6.1.E.2.68.ftf
09/032/2012 15:32:56 - INFO - Preparing files for flashing
09/033/2012 15:33:02 - INFO - Please connect your device into flashmode.
09/033/2012 15:33:03 - INFO - Opening device for R/W
09/033/2012 15:33:03 - INFO - Reading device information
I managed to flash the FTF with flashtool, when i unchecked wipe data, wipe cache, wipe app logs- i got my stock software back yeiii!!! But happy times did not last long, the phone calling feature did not work and contacts crashed, so i was unable to recieve or make calls, i did clean dalwik cache and cleaned cache of many com.androidxxxx processes but no success...
I was pretty angry that when i thought everything was fixed and back to normal, i have no calling feature on the phone, what makes the phone useless brick. I tried to reflash original FTF with flashtool but it was not flashing it! Next step was idiotic step i thought if i flash another kernel via fastboot i will be able to flash my phone to normal...
I did flash unsecure kernel on the phone
http://forum.xda-developers.com/showthread.php?t=1751065 and after that again Bootloop Sony logo
I tried to get it into flash mode and flash, but phone did not respond to flash request- so i was stuck with boot looped xperia for a while until phone was DEAD AGAIN after hard reset... It took 1 day for the phone to discharge and it STARTED AGAIN (boot loop).
I decided there are not many options i can do (as flashtool does not flash FTF, only flashes kernels in fastboot mode(if you are lucky enough to get into that mode)+ and i had CWM working
So i redownloaded paranoidandroid rom zip and google apps, installed them with CWM, and i was enough lucky to get the phone into fastboot (using hard reset + quick connecting usb cable)
I flashed 1.73GHz kernel successfully and my phone was working again, but in this damn paranoidandroid, what is not a bad rom (much smoother than original ics) but have some limitations..
So this is my story of Xperia ION modding, i do believe that sony is far behind competitors (HTC, SAMSUNG) when it comes to flashing and installing kernels and custom roms... If something will go wrong (and in my case it went wrong every time) you maby will be stuck with bootlooping device forever
I will wait until sony will release their jelly bean and then il try to flash jelly bean FTF with flashtool again.. but for now i just hope that paranoidandroid team will fix HDMI, choppy camera performance etc... because i am literally stuck with this ROM

))
Thanks for reading and i hope that it was helpful for somebody who is really desperate about his xperia ion being alive again, dont give up!!!