Wirelessly posted (8700g: BlackBerry8700/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100)

If you didn't get the confirmation window pop up at the t-mobile web page, nor the confirmation message to the blackberry, then it appears something didn't work. To be sure, you can look at options > advanced > service book and see if you have the two @yahoo.com service books. Also look at the t-mobile BIS page again and see if it shows your yahoo account or not.