New iOSes are typically distributed to developers early. So I have been able to test compatibilities / incompatibilites for some weeks already. The real device is a different thing, they are not available outside Apple ahead of release. New screen ratios are not public ahead of release, so most developers start now adopting to it. Typically, new devices do not break operation as Apple takes care to stay backwards compatible for existing apps.
The new screen ratio is effort for us developers. Old apps run in a 'letterbox' and leave the new 176x640 pixels black. As this is pretty ugly, I have finished full use of the upcoming iPhone 5 extra space today. Approximately 2 days of effort for a complex thing like LT. But it look great and will be beneficial for LT users.
I have posted a sample screen here: viewtopic.php?f=11&t=848#p4124
Comparing it to the iPhone4, there is extra space for two rows of gadgets. The concept will be that the tab bar will not hide in portrait any more but show an extra row of data.