Update 2: Apparently to qualify you simply need to use the T68i, you don’t even need to have the IMEI swapped on the account to get the T226 upgraded. Just drop the SIM in your old T68i and if you qualify you will get a letter in the mail.
Update: AT&T and Sony Ericsson have set up a web site here for the T226 upgrade program specifically, giving a comparison of the two phones, as well as how to move your contacts from one phone to the other.
Some of this may not be 100% valid, as it varies from area-to-area.
Due to AT&T now offering service on two bands, 1900 MHz and 850 MHz, AT&T has decided to upgrade anyone with a 1900 MHz-only phone for free. For example, owners of the Sony Ericsson T68i will be mailed instructions on how to upgrade to a T226 with a one-year renewal of their agreement. Other models that are 1900 MHz-only will be offered the Nokia 3100 or 3200 as an upgrade.
It looks like this is a trial program, being run in areas that are heavily covered by the 850 MHz band. Currently the program appears to only be offered in NY, NJ, and CT, as well as Los Angeles and San Francisco markets.