I don't know if I have caused this problem or something changed between B2TR and RTM, BUT:
Last tuesday while in the middle of a live demo at Microsoft's in The Netherlands I decided to create a mysite for the administrator. All of a sudden I was confronted with the error: 'Your personal site cannot be created because Self-Service Site Creation is not enabled. Contact your site administrator for more information.'
luckily I thought I saw a menu-item in Central Administration which was called 'site creation…something'. So I opened up Central Administration and in fact, at the Application Management tab I found the menu item called 'Self-Service Site Management'. Setting the option 'Enable Self-Service Site creation' (for the web application hosting the mysites) to 'allow' solved this error.
Phew….That saved my demo, so I didn't think anything more about it. Until this morning I got a call from a project team at Info Supports telling me they got this exact same problem. Same fix worked for them as well…but does anyone know what's causing this?