I've got WP 3.0.5 running in a subdomain install, with the domain mapping plugin. It's been working fine. I created up 5 blogs initially, and then installed the subdomain mapping plugin, and got it working fine. Several weeks passed, with writing and plugin isntallation/removal and all the other little changes that take place over time, and now I'm trying to create another new site, and it's failing.
When I look at the DB tables, the information for the new site is showing up in the Network wide tables (wp_blogs, etc) but it's failing to create the new wp_N_posts, wp_N_comments, (etc) tables that would hold the content for that particular blog. The DB user has all privileges on the table of the blog DB, and I don't see any explicit errors showing up in the debug.log pertaining to the table creation -- though a huge number of them appear when it tries to actually *do* anything with those (non-existent) tables.