The guy over at thekid.me.uk has posted a v. handy little note about how to get your a servers control into the toolbox in Sharepoint Designer.
Apart from registering your control within the SafeControls part of the web.config), the secret sauce is to add :
[ToolboxData(“<{0}:ControlName runat=server />”)]
to your class (just above the class declaration)
and
[assembly: TagPrefix(“yournamespace”, “selected prefix”)]
What then happens when you drag the control into designer is that it inserts
<prefix:ControlName runat=server /> into the page.