Posterous has offered support for custom domains for some time now, but it still required the user registering a domain with a registrar and then altering their A records with the registrar to point to Posterous.
Like WordPress.com, Posterous users can now avoid that step. You can check the availability of your desired domain name and if it's not taken, register it and have it automatically mapped to your account in one step.
Furthermore, you can set up Google Apps for your domain (so you can have email, calendars and access to all of the Google Apps Marketplace stuff) really quickly. This is pretty useful, because while some registrars will let you set up Google Apps alongside your domain, this will take care of that extra step of also hooking it up to Posterous.
Posterous says that for the first month, the price of domain registration through them will be discounted. Currently, you can register a domain for $24.99 for one year or $19.99 per year if you register for two years. The normal registration prices weren't revealed. You can only register .com, .net or .org domains through Posterous.