I just notice the same issue on two different new blogs/subdomains. Once the new blog is created, Random Theme on Signup should assign a random theme to it. Instead, the new blogs get no theme at all. On the front end is just white, and in the backend is Active Theme: [blank space]. I must say I have tried all the themes on the WPMU packages, but after I deactivated most of them - Now I only have active 6-7 themes, and just one of those is from WPMU package.
Hope someone can help me. Thank you