The bulk of the merge has been done in WordPress core. At least with regards to moving over the files. Now the fun part begins: bug gardening, testing and tweaking.
Here’s what you can do to help not only the dev team and others, but also yourself:
– set up a local install of WPMU
– import some data, plugins & themes from your current setup
(in other words, create a working copy somewhere)
Now, upgrade it to the latest WordPress trunk. The zip is located at the bottom of this page. To manually upgrade, just overwrite the MU files with the files located in the zip.
When you find bugs, please file them over at the WordPress trac, and pick the term multisite in the component field dropdown, so the other devs know is it MU related. Some terminology is changing, so this refers to the multiple blog capabilities. Your login to file a ticket is the same as your forum username and password.
Did you get the part where I said to do this on a dev version and not a live site? Good. 🙂