We had a pretty clear indication that a carrier update would soon be available to help bring unlocked GSM iPhones onto T-Mobile's network, and those leaks were indeed spot on. The carrier announced today that just such an update is now available, enabling Visual Voicemail and other features (included a promised battery life improvement) on all unlocked GSM iPhones running iOS 6.1, while unlocked GSM iPhone 5s will also get HD Voice and T-Mobile LTE support. Complete details on the update -- which can be installed manually or over-the-air -- can be found at the source link below.
Update: It looks like quite a few folks still aren't seeing the update just yet. Let us know in the comments if you are able to snag it.
Update 4/8: After a rather quiet weekend, it looks like most are now finally receiving the update today.