I recently had the need to migrate a standard WP 2.2 blog into WPMU, with the requirement for the "root" blog to contain the content from the original standard blog.
This worked just fine using the import/export features of both, exporting from 2.2 and importing into the main blog. The only catch was that MU adds the /blog/... to the permalink structure of the main blog, and the old permalinks (just date based: "/2007/..."), so old references would not work in the new installation.
I managed to add a rewrite rule that would support either format and direct it to the right place, so that /blog/2007/... AND /2007/... both go to the same target, and it seems to be working fine. I can post the code I used (in the form of a plugin) if anyone's interested.
My question, for those more experienced in MU than I, is whether this will cause any problems I haven't uncovered yet. Like I said, so far, so good, but I just wanted to run it by some people with more experience to see if I'm missing anything that will come back to bite me later.