Before you panic, you should know that this isn't a huge deal, and Comcast is aware of the situation and has promised a fix "within a week or two." There, feel better? Good, because if you use the XFINITY app, any other app that has permission to read logs can read your Comcast username and password (aLogCat, for example).
The details, courtesy of aBSuRDiST, who discovered the issue:
My system log shows <userName>[email protected]</userName> and <password>MYPASSWORD</password> on a line that starts with "D/HTTPManager".