I started implementing a control for this a while back, and finished it today, it should be in svn trunk by late tonight.
You can add it to layout.master like this:
<span style="color:White;font-weight:bold;"><portal:PageLastModified ID="pageLastMod" runat="server" Visible="false" /></span>
of course you can style it as you wish, the above example is for styleshout-brightsideoflife skin
If you have the current svn trunk, the control actually exists already there but you need to add it to Web.config <pages><controls> like this:
<add tagPrefix="portal" tagName="PageLastModified" src="~/Controls/PageLastModified.ascx" />
It will only show the date using the one currently in svn trunk, but once you get the new version it will show a label as well and has a few properties you can configure like LabelText and ShowTime.
Also if you use the same id as I did, "pageLastMod", you can set Visible to false and this will hide it on non-content system pages. Content Pages are pages in the menu, they are all handled by Default.aspx.cs and there we look for the control with this id and make it visible if it exists.
Hope it helps,