I currently have a number of installs running the Domain Mapping plugin. I just attempted to setup another install using the plugin and I absolutely cannot get it to work. It seems like there is something going wrong in the generation of the necessary database tables. I went ahead and compared two other installs with this one and I noticed that this install is using wp_domain_map as the mapping table while the others are both using wp_domain_mapping.
@Barry, I'll create administrative credentials for you if you'd like to see this install for troubleshooting purposes. I know this topic has cropped up before and it might be useful to get diagnostics off a fresh install which is exhibiting issues.