SEO with Multi-Domain & Domain Mapping Plugins

Hi,
The new Multi-domains plugin has given me the idea of merging several small WP3.0.1 multisite installs with separate domains, into one large multisite (very few members right now, so easy to move the blogs). What, if any, are the SEO implications when using the Multi-Domains and the Domain Mapping Pluggins in a multisite environment? My users will definitely want to promote their sites.

Any issues with other WPMU-DEV plugins? Supporter? Multi-DB?

Thank you,
Chuck S.