A handset launching towards the end of the 1st quarter 07 would already be undergoing testing and development by the networks it's launching on.
Only if it's being sold by those networks. If it's being sold SIM free, the networks don't really need to care. And, while it may be true that some CDMA2000 networks will block "unknown" phones, that just doesn't happen in the GSM/UMTS world, with the exception of the examples I gave.
Whether this is the same for the US market I'm not sure but in my experience it's the case in the UK. In regards to blocking, if an Apple phone poses any problem for the networks they can block IMEI ranges and say it's unsupported on their network. An example of this in practise would be - you take a 3G handset and put it's USIM into a 2G handset not sold by the network, within a few days you will get a message warning you the handset is unsupported and will be blocked.
Has this actually happened to you? If you're roaming, you may want to use a 2G handset. The issue here, in any case, is the person has an incompatible phone given their priceplan (and, in the case of UMTS-only networks like Three, their network.) The type of technology, not the phone itself is the cause of the blockage.
For the timescales and internal testing involved, a new handset that is identical to a currently sold one, but in a different colour, will often have to go through all of the internal testing again for power outputs etc, because of varying metal content within the paints, a totally new handset will take quite some time to be officially supported. I'm sure some networks aren't concerned with any old phone working on their network but there are some that definitely are.
Again, this really doesn't happen. Sure, GSM operators will fully test a phone before selling it themselves, but this is entirely different to blocking phones that they haven't tested. In some countries, notably all of those in the EU, this would be illegal as it violates the personal mobility part of the GSM mandate.
The only instance I can see where a regular GSM operator would block a regular GSM phone is if the phone itself has a broken design and doesn't meet the GSM specification. Likewise, the only instance I can see where a regular UMTS operator would block a regular UMTS phone is if the phone itself has a broken design and doesn't meet the UMTS specification for their supported AI technology (W-CDMA, TD-CDMA)
Blocking a phone because they haven't tested it themselves is a surefire way to have the EU kick them. It serves no purpose at all, and prevents people from being able to use the technology of their chosing. Realistically, a GSM iPhone, whether it's sold by an operator or bought SIM free directly from Apple will, unless Apple actually has flaws in the fundamental design, not be blocked by any GSM operator.