Ah, yet more comments based on speculation.
Facts from German and English-language forums, and of course, usage of the iPhone myself.
First: You need to activate it ONCE. This is in iTunes. If, like me, you choose to jailbreak it, the only reason to use iTunes is to sync or update the firmware; I cannot verify how updated firmware behaves regarding activation, but I have removed and replaced the O2 PayG SIM in mine with no issues, and the only difference is a patched lockdownd.
Second: The unlocked phones can have SIMs changed freely according to reports from users.
iPhone is not terrible value - you do, after all, need to pay for your calls and data use somehow. It simply isn't the best deal available and that is irritating, because it is an excellent device - very easy to use, very powerful and responsive for something so simplified. Compared to a WM6 phone it is exceptionally quick to use, even if it drops some features.
Anyone paying 999 Euro for it is an idiot. Pure and simple. However, in Germany the sales model differs yet again; so I'll do a quick outline:
USA - AT&T, phone is bought in AT&T or Apple stores without a contract. 2 year contracts, start price for contract is $59/month, handset cost $399 - 450 minutes, 200 texts, unlimited data.
UK - O2, phone is bought in O2, Carphone Warehouse or Apple stores. CPW make you sign a 'contract' which is utterly meaningless. 18 month contracts. Handset costs £269, £35/month 200 min/200 text, unlimited data, free WiFi access with The Cloud.
Germany - T-Mobile, phone is bought through T-Mobile with contract handled in store (as far as I can tell without going through the process myself). 2 year contract, entry level is 49 Euro/month, 100 minutes, 40 texts. Data may be subject to fair-use limits. Free WiFi access with T-Mobile.
Germans cannot easily buy a local-market phone and take it home, jailbreak and "hacktivate" it. They also get scalped horribly on the handset cost and the contract.
Apple's iPhone agenda ends with them marketing an Apple-branded network based on capacity and services purchased from existing carriers, billed and paid via iTunes, and significantly cheaper than existing deals. I'd put money on it, if I had any left after buying an iPhone...
Already this exclusivity