I'm not sure if it's an all things root or a normal post, but I'm posting it here, anyway.
Yesterday I made what every nOOb, like me, fears the most. Messing with the ROM Manager app, I made some mistake (not sure what it was, as I followed the instructions to the letter) and bricked my phone. It was stuck on the fastboot screen and responded to nothing.
So I ransacked the Interwebz searching for the information I needed for a solution, which was scattered in a dozen different sites and forums and I'll try to gather it all here for future reference.
1 - If you bricked your phone with ROM MANAGER (stuck in Fastboot mode screen), or any other root app, download the KDZ_FW_UPD.RAR LG flash utility to your Windows, the V10C_00.KDZ (latest LG P-350 firmware), and LG_United_Mobile_Driver. Turn off the handset and take off the SD Card.
2 - Install the United Mobile driver from LG on Windows, then go to control panel and disable the modem driver. If you don't disable the LG modem driver, the damn thing will not work.
3 - Unpack the KDZ_FW_UPD.RAR to a new directory and install, first of all, the msxml.msi program.
4 - Make sure your battery have 100% load.
If it doesn't, wait at least 2 hours before trying the following step and let the battery charge (keep it charging).
5 - Take off the battery and the USB charger from the phone.
6 - Plug the USB charger and keep pressing the PLUS AND MINUS VOLUME buttons. Making this will start the phone in a cheesy yellow screen with an "Emergency Mode!!" warning. Put the 100% loaded battery back on the phone.
7 - Start the KDZ_FW_UPD.exe program on the PC.
8 - Once KDZ_FW_UPD.EXE is running, chose the Type 3GQCT and PhoneMode Emergency on the respective drop down menus.
9 - Chose the V10C_00.KDZ firmware image on the KDZ file option.
10 - Click on the Launch Software Update button, hope for the best and go grab a cup of coffee or tea.
11 - Fifteen to twenty minutes later, if the above procedure worked, an animation asking you to close the phone will appear: Your phone is saved. But don't close the phone yet.
12 - Start the phone again. If you get stuck in a screen with an Android and an exclamation mark ("!") in a triangle, just unplug the USB cable and the battery. Plug the battery again and start the phone. It will take 2 to 5 minutes to reboot for the first time and the boot animation will be the stock LG (your carrier's stock animation is lost for good), but chances are your phone is intact, with all it's apps and configurations working the same as before the incident. When the boot finishes, turn off the phone again and put back the SD Card you took off on the first step.
13 - Erase ROM MANAGER from your P350.
There is probably a 100 easier ways to make this, but that's the only way it worked for my phone - and I tried hundreds of other things. As soon as I can post links on the forum, I'll put the links to this utilities and the firmware on this same thread.
Cheers to all,
EDIT: There are some slight differences between my original firmware and this (newer) one. There is a couple of new system apps from LG and the phone looks like a bit faster compared with the original firmware. And I forgot to mention that the root privileges go away, but it's possible to reroot the P350 again using gingerbreak.