This was unexpected, and not documented at all in the SharePoint SDK. When you create a UserMulti column, you must also specify Mult="TRUE", otherwise it will fall back to a standard (single select) User column type. <Field ID="{12312312-1234-1234-1234-123412341234}" … Group="My Group" Type="UserMulti" Mult="TRUE" …
In order to tell which databases your SharePoint 2007 / WSS 3.0 installation uses, you will have to look at multiple locations. Content databases Open Central Administration and go to Application Management and then Content databases Review for all Site Collection URLs Shared Services Databases Open Central Administration, Application Management, Manage this Farm’s Shared Services [...]
If you have a field in SharePoint that is setup as a choice field, and you want to have multiple default values, you can do the following: Instead of selecting the Default option, select the Calculated Value and enter the items separated with semicolon and the hash sign. Like this: =";#Red;#Green;#Blue;#"