iPhone SIM swapping question

Hey all, so, I have an iPhone 3GS, that I've been using since launch, but recently I broke a ribbon cable trying to fix my home button (button works now though, lol). I didn't feel like buying a replacement for it because why spend money on a 2 year old phone.
I saw this opportunity to try out an Android phone for the time being until the next iPhone comes out. I bought a Nexus S, but there are some things I miss about my iPhone, so I thought I could switch phones every few days or so, but when I put my SIM into the iPhone from the Nexus, I was able to make calls, and texts, but I couldn't use data, and my plan stayed the same, I still have my Unlimited from when I bought the 3GS, I made sure it would stay that way, and they assured me it would, so does anyone know why this is happening?