I am moving over from an Acer Liquid to a SGS2 and am trying to get my head round all the new ins and outs of the device.
What is this "counter" that everyone keeps going on about and how does it affect me?
The counter records the number of times you "download" and use custom firmware. It may invalidate your warranty to use custom firmware and the counter makes it difficult to hide as it indicates that unofficial firmware has been used even if you re flash the official the official firmware.
A warning triangle appears when you boot your phone when you have a custom ROM installed (it is removed if you reflash the office ROM).
Not a big problem though just buy or make the usb with resistor it will remove the yellow triangle and reset the count to zero.
You can also remove the yellow triangle by reflashing the secure firmware after you have rooted and this also changes the firmwares designation back to Samsung Official in Odin Mode.
Follow the guides and you will figure it all out. They also have some good stuff over at xda.
Hope that helps a little.