Tumblr appears to be the most notable import option of the bunch, because they directly compete with Posterous in the middle space between blogging and microblogging. But Posterous seems to be taking aim at the larger pool of blogging services too and positioning themselves in the same light. Essentially, they're throwing down the gauntlet, and giving bloggers across a myriad of platforms an easy way to make a clean break.
Just like pretty much everything with Posterous, importing content from another blog — Blogger, Tumblr, WordPress, TypePad, Movable Type, and Xanga — is dead simple. Basically all you need to know is your URL and respective username and password. Then you can opt to merge the entire site or just individual posts, and the rest is automated by Posterous.