Is it possible to create a .CSS file for each SharePoint Page Layout I develop, or does the CSS for each possible layout in a master page need to be referenced in the master page?
IE is it possible to affect the <head>
of the page a page layout is used in?
Michal's solution can be further enhanced by including any links etc in the PlaceHolderAdditionalPageHead content placeholder tag on your layout page. This way it will be included properly in the head of the generated page.
e.g.
<asp:Content ContentPlaceholderID="PlaceHolderAdditionalPageHead" runat="server">
<link id="Link1" href="<% $SPUrl:~SiteCollection/Style Library/mystyle.css%>" runat="server" type="text/css" rel="stylesheet" />
<SharePoint:ScriptLink id="jQueryCore" language="javascript" name="ui.core.js" runat="server"/>
</asp:Content>