I also have X10a with AT&T
and they wont release 2.1.
The way i updated to 2.1 i downloaded the stuff from this guys video look in the info for links.
http://www.youtube.com/watch?v=85-WbEupcyg
Make sure you put both in separate folders when extracting.
1.Download both files and put part 1 and part 2 in separate folders.
2.Download SEUS (Sonny Ericsson Update Service) and run follow the steps and let it update your drives (If you haven't already).Also download PC Companion and run.
Both can be found here
Sony Ericsson - Sony Ericsson
3.After all that plug in phone open and run SEUS again once you get to the point were it tells you your already up to date cancel and exit DO NOT INSTALL.(KEEP PHONE PLUGGED IN SHOULD HAVE A CONSTANT GREEN LED)
4.Open part 1 that you downloaded from the video and click on X10flash.bat Go threw the steps like so
a.y
b.1
c.5
d.y (MAKE SURE YOU PUT Y IF YOUR ON 1.6)
f.This depends on what your operating on select accordingly.
5.Then let it run it should detect your X10 and and start flashing it will take about 3 minutes when its done press any button and unplug your phone hold the power button till it vibrates then wait some more you should see the SE logo followed buy a camera and a blue arrow it will take about 3 min for it to finish.
Then your done you will loose apps,phone numbers and text messages and your root NOTHING on your sd card so i would save your contacts somewhere.
6.If you dont want the rogers crap when you boot then Open part 2 and run the .bat file in their go thru steps 2-3 on step 4 slect the following
a.y
b.1
c.3
d.n
f.This depends on what your operating on select accordingly.
Then just boot up and your done
It will not erase anything so if youve already started getting stuff back dont worry.
If you want to root again follow these steps i just posted here
http://androidforums.com/x10-all-things-root/232271-step-step-rooting.html#post1967393
Hope this helps.