Dear Google Juice,
This morning I tried to enable the security stuff with my replacement phone. I entered the default pin of 8 0's like T-Mobile's website and Customer Care people told me to, but my phone went into "PIN Blocked" mode and I was unable to use it at all. T-Mobile Customer Care was absolutely useless. They kept insisting that my phone should prompt me for a PUK Code, which it didn't. In the end, they told me I was screwed and that I needed to buy a new SIM.
Turns out the Samsung T619 doesn't have the ability to unlock a SIM card, but you can easily unlock it using a friend's phone (preferably not a Samsung). Just get the PUK code from Customer Care, put your SIM card into your friend's phone, and power it on. The other phone should prompt you for the PUK code, which'll let you set a new PIN as well. Switch your SIM card back to the T619 and you're set.