1. I hope we add this soon
2. I'm going to take my chances among the HN community and say: I think this is low priority because
i. Most other services don't even offer import. Posterous offers import from (soon to be) 15+ services. this is not easy
ii. Posterous supports all different post/media types so that we can import from so many different services. We put our focus here
iii. I care about sharing with my family and friends. They don't care about permanent URLs as much as being able to find my site and search for what they want. cc/google
If you are a pro blogger (and maybe you are), and you have major google juice on some URLs, then yes, maybe you care about this. But most people aren't, and don't.
There are many reasons why someone should or shouldn't move between competing services on the internet. But urls seems like a very small reason to make that decision. Future services (Posterous or maybe others!) might offer super compelling reasons to move.
Not moving because of urls seems like not switching to a Mac because you have all the Windows keyboard shortcuts memorized.
In my lifetime, I expect to move between blogging platforms, phone providers, desktop OSs. I will buy different brands of cars, live in different neighborhoods, probably even switch email providers.
Yes, there are switching costs. But we hope to minimize them. And more importantly, we hope to focus on adding awesome features so you are willing to incur that switching cost.