Share your experience!
While running the 2.1 update on my phone the upgrade stopped.
Now i can't start the phone, a warning icon shows on the display for a couple of seconds and then the phone turns off again.
Please help ASAP!
Try repairing the phone using SEUS. Before that make sure that all firewall and antivirus applications on your PC is stopped. SEUS allows you to update the phone when switched off. So go and try the repaie with SEUS.
Have tried that, didn't work 😞
What happens when you try to repair using SEUS?
I get the error message: "Update failed, try again"
At what stage of the repair do you get the error message?
After the upgrade is finalized, 100%
Do the following.
Switch off the phone.
Remove the battery, sim card and SD Card.
Reinsert them.
Keep it switched off.
Uninstall the present version of SEUS from your PC
Download the latest version of SEUS from http://www.sonyericsson.com/cws/support/softwaredownloads/detailed/updateservice/xperiax10?cc=in&lc=...
Do steps as instructed on screen by the SEUS software
When SEUS asks you to connect the phone, without switching on keep pressing back button until the software asks you to release the key.
Wait until the download and update is complete.
Thanks! Will try that later, I am at work with firewalls etc...
Have a nice weekend! 
I am also experiencing the identical issue to the original poster: The 1.6 -> 2.1 update failed and my phone will no longer turn on (exclamation mark in yellow triangle).
Following the instructions above, I uninstalled all Sony Ericsson software (PC Companion and Update Service) and tried the update again using the version of SEUS from the link provided above, but I still get the same end result.
My X10 is direct from Rogers in Canada, and I have never done anything "experimental" with it (e.g. XDA updates or rooting of the phone). Should I be looking at returning the phone to Rogers, calling SE tech support (which I plan to do tomorrow morning), or is there some other option to try? I saw mention of an XDA 'unbrick' flash technique in this other thread, but am very hesitant to try that in case it causes my phone to become unsupported. I would rather go through the official channels first but I hope they will be responsive.
Frustrated, but exercising patience...