A post on the Sony Ericsson Product Blog announced on Saturday that some variants of the Xperia X8 handset are ready to receive the Android 2.1 update via an OTA update. The update will be a gradual rollout, so don't be surprised if your phone is not already listed.
Due to the nature of peforming rollouts in different countries with different providers, the exact dates for when a certain variant will receive the Android 2.1 update cannot be announced until all necessary approvals have been received. Once those approvals have been received, the updates are released.
If you don't see it today, check back every couple of days (or more often if you can't wait) to see what new numbers are now supported. The new additions do not release at some pre-determined time, so checking back regularly might be the best way to get your update ASAP.
In the blog post is a large list of "SI-numbers" for phones that can download the Android 2.1 update. To find the SI-number on your phone, look at the label on the back of your phone and cross-reference it with the list.
Head over to the Sony Ericsson Product Blog at the source link to see the extensive list of Xperia phones that are already available to have the update.
Image from EuroDroid