I think people might have been getting confused with the dish and the LNB.
[geek]
There's two parts to the "dish" - the dish itself and the bit that actually takes the signals in- this is the LNB (Low Noise Bloc), and is the black thing at the end of the arm.
Each individual tuner requires its own LNB, because it's that which locks onto a frequency. So when you tune to BBC1, a signal is sent down the cable to the LNB which tells it to lock onto
such and such a frequency.
For Sky+, you'll notice there's 2 cables. This is because you need 2 LNB's so that different channels can be recorded. Both are housed in the one bit of plastic, so there's just the one thing on the end of the dish arm. Some people (like us!) have a quad LNB, which (funnily enough) gives 4 outputs for 4 tuners. You can get octo ones as well.
I won't bore you with details about quattro LNB's and multiswitches!
[/geek]