HTC Desire :: Wireless Hotspot Feature Works Without Data Plan

I purchased a HTC Desire a few days ago and I think it's great. However, there is one thing that bothers me. I've been quite slow at joining the smartphone community, primarily due to all the reports/stories of high phonebills relating to internet usage. So, having got my hands on the phone its self - I thought I'd play with it for a while and stick to using my home wifi connection for downloading apps and playing with map applications etc.

The phone came pre-installed with Android 2.2 and therefore has this wireless hotspot app built in. So, to see how it worked, I enabled the feature and connected to the phone from my laptop.

My first surprise was that it worked. My laptop was connected to the phone and web pages were loading up on it. Realising that my phone was already set up to connect to my wireless router for internet access though, I figured that it was probably sharing that connection with my laptop. Well, at least that's what I was hoping...

At this point I unplugged my home modem/router and tried again. Fully expecting my laptop to complain about the lack of internet connectivity, I was quite shocked to find that web pages were loading up on my laptop and there's now a '3G' logo (with flashing up/down arrows) showing on the top status bar of the phone.

From what I've said here, my only conclusion is that my phone is transferring data over the mobile phone network, but considering that I don't have a data plan, how can this be? If I disable the hotspot feature (with my home router switched off), my phone cannot access the internet, but if I enable the wireless hotspot feature of the phone and connect to it - my laptop can.

My SIM card is from Tchibo Germany (o2 network).