In ConfigMgr 2007 or 2012 the following error message can be displayed during boot image loading: "Program Files cannot be located on a Distribution point". This error message (which is still valid in ConfigMgr 2012) can be passed with the following: Select "When no local distribution point is available, use a remote distribution point" in the advertisement/deployment.
I think it's because the client is on a different subnet then the ConfigMgr server/Distribution Point? Anyway, this helps!!