I'm not familiar with Posterous, so I'll describe a more WordPress oriented solution that needs access to your own hosting server. (I assume you have this already since you want to install WordPress). From what I have understood, the steps would be as follows:
1) On your hosting space, set up a redirect using .htaccess, that will redirect visitors to your Posterous account or pages, except if you try to access a specific subdirectory. Don't think you need a subdomain for this particular situation. Read up on how to do an .htaccess file, but specific instruction for this redirection trick can be found here:
2) Get your domain to now point at your new hosting account instead of Posterous. If the .htaccess trick worked, all your old urls will continue to work as normal.
3) Setup WordPress in a subdirectory, the same one you set the .htaccess redirect rule to ignore. This way you can continue to setup and work on your new WordPress site at domain.com/wordpress/, while your Posterous site remains accessible.
4) Once all data has been moved to the WordPress install and you're ready to switch, get rid of the .htaccess redirect, and follow the instructions to make WordPress work in the subdirectory.
5) Make sure your Permalink structure in WordPress matches your old Posterous site and everything should work seamlessly, with no 404s or not-found errors from old links.
Hope this helps,