I'm not familiar with any plans of ERA rolling out 1.5 software anytime soon... Do you have any links to that info?
If you wanna play with 1.5, get The Dude's Cupcake at
It includes polish locale and works quite flawlessly
The procedure for unrooting the phone would presumably be very similar to the one you've already found here on the forums:
- get the original SPL
- rename it to update.zip and flash it to your phone
- get ERA system image (renamed to DREAIMG.nbh, I believe) on the root of your SD card and start phone in bootloader mode (camera+power)
- hit power to start the update
- hit action(trackball down) and your phone will reboot
AFAIK you could use the files from sticky threads in this forum, the T1 kind. ERA update files should still be recieved automatically and update the phone anyway.
As for the SPK/system image files themselves - if you find any original ERA files, please post em here