Congstar customers who want to switch to the iPhone 5 right at the start are currently looking at the tube: according to the corresponding Support threads in the Congstar Forum the Telekom subsidiary will not offer Nano-SIMs for a few months, in some places the talk is of 2013. In fact, Congstar is supporting a mobile phone that has a ETSI-certified SIM card standard not for the time being. As a customer, however, I would expect that a mobile phone provider would also support all devices that comply with current standards - and force customers who don't want to change to a new one to cut off something on the SIM card (even though there are now templates for this: NanoSIM template).
It is noticeable that Congstar, as a Telekom subsidiary, has "problems" with providing NanoSIMs for the iPhone 5 launch. There is a suspicion that the parent company wants to see its sales of such contracts (and iPhone 5) secured before a cheaper version is available on its own network. After all, Congstar offers considerable data volumes, which could also be used with the iPhone 5. Perhaps they will be able to see the point and bring the nano-SIMs earlier than expected - customers, even those without an iPhone 5, will certainly appreciate this. Because: "You want it. You can have it." - That should also apply even if it's not ideal for the provider.
Update: Congstar announced today that at least post-paid customers will be able to order a Nano-SIM free of charge from 15.10.2012. New customers will still receive micro-SIMs, but will also be able to order nano-SIMs for iPhone 5 free of charge.