Is there any way to reference data that is located in another subsite?
Not without building a custom field type. Out of the box Lookup columns arestrictly within the current site.
spservices.codeplex.com/<http://spservices.codeplex.com/>Specifically:http://spservices.codeplex.com/wikipage?title=$().SPServices.SPCascadeDropdowns&\referringTitle=DocumentationIt's for cascading dropdowns but you most certainly can select the site thatcontains the list you want and only use one Lookup Column.
You could use CQWP (Content Query Web Part) or you could use SPSiteDataQueryif you are using code.
You could also use SharePoint Designer to create a Web Service Connection tothe list.http://www.youtube.com/watch?v=5qthY6iMEbsThis also works across site collections and even across server farms.