Firmware update from my carrier Cosmote iPhone

Hi,
When I connect my iPhone to iTunes, I have a message which asks me if i want to install on my iPhone a new firmware update from my carrier (which is COSMOTE - Greece).
I would like to know what does this update, I am afraid it could block free internet tethering or Skype calls through 3G networks.
I have another question: is it possible to permanently disable firmware updates in iTunes? (Because one day I could make a mistake and click on “Yes”)
Thanks,
Eric
iPhone 3GS iOS 4 iMac + MacBook Ai