WordPress.org

What about sub-categories?

WordPress doesn't even support sub-categories for link categories. If you try to upload an OPML file that is sub-categorized, you can probably expect strange behavior. However, depending on the structure of the file, you may still be pleased with the result.

Which systems that generate OPML files have you tested?

So far, I have tested WordPress generated OPML files, and Firefox (with the OPML Support add-on) generated OPML files. If anyone has trouble with an OPML file from a different program, let me know all about it and I'll see if it is an easy fix.

Is the category check case-sensitive?

No. the category check is case-insensitive. Therefore, you could have a link category named 'Cars' in your blog and a category named 'CARS' in the OPML file, and those links would be properly inserted into 'Cars'.

Does this plugin work with WordPress versions less than 2.7?

No. At least not yet. I built it on 2.7 and poked around trying to get it work on 2.6 but did not succeed. If people actually download this plugin, maybe I'll work on some backwards compatibility.

Can this be a mu-plugin in WordPress MU?

Yes, you'll can put the main plugin file import-blogroll-with-categories.php in the mu-plugins directory to enable it automatically for all blogs. You'll just have to put the file bwc_opml.php in a sub-directory and change the path of the include in the set_bwc_opml() function.