Re: WPMU = WP
Yep. Single WP could have multiple users all along.
To answer the OP, if you want to start building on WPMU now, go right ahead. You won’t need to re-do anything after the merge. The hope is the upgrade will be quite smooth. You’ll just be upgrading to the “regualr” WP codebase, but your multiple blogs are alreayd turned on in wp-config.
If anything, it’s single WP going away.