ISIS, the joint mobile payment venture backed by AT&T (T), Verizon (VZ) and T-Mobile, was previously set to debut in select markets at the end of September, but undisclosed delays continued to push the launch date back. According to a purported internal memo obtained by TmoNews, however, it looks as if the service will launch on a trial basis on October 22nd for select T-Mobile customers in Salt Lake City, Utah and Austin, Texas. In order to use the Mobile Wallet service, users will need a special ISIS SIM card that will be compatible with the Galaxy S III, Galaxy S II and Galaxy S Relay 4G. It should be noted that rooted devices will not support the service, just as rooted devices are unable to use Google Wallet. We’ve posted a second image after the break.