In March, on Twitter, I said the following: "http://squarespace.com is kind of blowing my mind, but all of that control and no way to modify permalink structure or htaccess? damn it!"
It looks like Squarespace's new import tool might do the trick, and as far as I know, Squarespace is the first of the hosted blogging platforms (e.g., Tumblr, WordPress.com, TypePad, Posterous, etc.) to provide this sort of feature. From the announcement:
For users with custom domains moving over, we actually use the URL structure of your existing site and create mappings for every single one of your old posts — automatically. This ensures that when you move your domain over to us, in most cases, all links to your old posts will work, and nothing will break.
Awesome! I'm curious to hear about real-world experiences with the new tool. The last time I moved between systems (Movable Type rarr; Wordpress), I decided to change my permalink structure — I removed the day from the URI — and had to come up with an automated way of mapping the old URIs to the new ones using htaccess rules, which rules persist to this day in my current (and very long) .htaccess file.