Finally I was able to get an iPhone 3G. I bought it from Manor.
According to toppreise.ch, you can buy the 16GB model it either with Swisscom prepaid SIM for 619.- or with Orange prepaid SIM for 899.-. But as I travel often and want to use my own network's SIM (which doesn't have roaming with neither yet), I bought the "APPLE iPhone 3G 16.0GB ohne Vertrag" (means without contract) for whooping 1199.- CHF (for your foreigners out there, 1 CHF is approx 1 USD). So I paid 580.- more so I have an universal iPhone and no prepaid SIM with it.
But what did I got? A SIM locked iPhone! So why did I pay 580.- more? Because they removed the prepaid sim from the bundle?
A total rip off.
As the Manor offering at toppreise.ch has been removed in the meantime, here some screenshoots of what it was before.