I have a user requirement to add a webpart inside another; I am creating a tabbed container webpart, inside which the user wishes to add other webparts. How can this be achieved?
If you have your own webpart it may be possible but you have to do all the plumbing your self. The standard webpart framework does not provide any thing for nesting webparts.
Otherwise you could try using Easy Tabs to add tabs to SharePoint, these tabs allow (multiple) webparts.