Make sure the phones battery is over 50% charged
Disconnect the USB from the phone
Turn off the phone
Wait 30 seconds
Press and hold the 'back' button
Plug in the USB
The LED on the phone should go green, and SEUS should go to the next stage and tell you it's okay to release the back button.
I've got a UK SIM-free X10 Mini Pro. I rooted my phone and altered my build.prop to Swedish, and updated to 2.1 yesterday morning. Then I had to re-root and alter the keyboard setting back to English.
After that, I've had no problems! Some things seemed weird at first.. The unlock key has changed to the middle button, the email/text notification only flashes the LED when the screen is off, and some of the program settings have moved about, but other than that, it's all fine! If I'm picky, Timescape seems a bit slower, but not much slower.
edit: found an issue!
Timescapes integration with contacts has been 'improved'.. unfortunately, it's made things worse, because it's added all my facebook and twitter contacts to my contact list, even ones with no phone numbers.
There are options to hide these contacts.. you can hide twitter contacts, you can hide facebook contacts, and you can hide contacts who don't have a phone number..
Unfortunately, that option doesn't work. I've selected all of the above, and nothing's changed - my contact list is a mess!