The first step in migrating away from Movable Type was to get all of my entries and comments in a structured format that could be parsed and uploaded to Blogger*. MT doesn't hold data hostage, there is a documented import/export format. Six Apart considers the format "lossy", in that it doesn't save a complete snapshot of your blog. I decided that what it did contain was good enough, though it turn out that what it lacked (entry IDs and permalinks) did make things slightly more difficult. A search on code.google.com for Python code to parse the format turned up Transfusion which does just that (searching for one of the magic strings in the format, CONVERT BREAKS specifically, was the easiest way to track this down).
As I was skimming through the exported entries, I saw that they weren't quite HTML. I had used MTMacro to create various shorthand tags for linking to entries, reference to images, etc. Similarly, I used MTCodeBeautifier to pretty-print code samples. None of these were getting evaluated when exporting, and even if they had been, I probably would have wanted to tweak their output anyway (e.g. to change URLs). Generally, it seemed like the time I had spent customizing my Movable Type installation with cruft-free URLs, plug-ins, etc. would be directly proportional to the time I would have to spend migrating away from it.
One of the more prevalent macros I had used was one of the form
<entryLink id="NNN">foo</entryLink> so that I could link to my past entries. Unfortunately, since entry IDs were not included entry IDs issue, there was no easy way to turn these into actual links, since the exported information did not contain entry IDs or URLs. In the end, I ended up converting these by hand.
That's it for the exporting part, part II will contain the Blogger import process and part III the template/design reasoning.
* the other migration option that I was considering was WordPress. However, the idea of having to do SQL queries to serve traffic didn't seem that appealing given my current provider's slow SQL performance. WordPress.com would have been a hosted option, but if I was going to relinquish control of the installation, it might as well be to a Google product.