You run into the same problem you had with Posterous. Having your own domain does nothing to prevent Tumblr from running out of cash and shutting down your blog. Luckily Twitter/Posterous are doing the right thing by providing a way to retain control of your data. Imagine if they didn't provide those tools, you're at the mercy of the platform.

I'm not on Posterous, but I am on Tumblr and I do worry the company won't always be around once the cash runs dry or they get acquihired.

I like the WordPress strategy of strong data portability features within a hosted environment and an open source, self-hosted option. This way even if WordPress.com (Automattic) gets acquired, WordPress.org will always be an option without having to get comfortable with an entirely new platform. Of coarse, if you're a developer you could always just use an open source static site generator like Jekyll or Middleman.