This is a funny one.
Site_old had a custom company logo in the site definition somewhere (not sure exactly where), but when it transferred across it was looking for that logo on site_old url.
I edited default.master to get around this.
Now I've noticed in UserDisp.aspx when displaying user info, the logo is on this form and its once again looking for the logo on old server.
What master page does UserDisp use?