Ask HN: How is Google rank affected by moving Posterous blog?

Now that Twitter has bought Posterous, and they will likely shut it down at some point, it makes sense to think about moving my blog to another service. If I have a Posterous blog on a custom domain, and it has a certain Google page rank, if I move the blog to another service and keep the custom domain, I assume that the Google page rank will remain the same. Is this correct? Is there anything I need to be careful about during the transition, so as not to get a lower rank in search results?