The basic situation is that I have a list of company contacts that exist in one site that I would like to share. The contacts are referenced from a list within that site, and I would also like to be able to reference them from a list in another site collection. Lookup columns can only lookup values from a list within a site, but this is the functionality that I would like across site collections.
Is there a common solution to this sort of requirement?
The SharePoint GUI only supports lookup columns in the current site. But if you create a normal lookup programmatically, or use one of many free cross-site lookup columns you can point across sites in the same site collection. I don’t think this will help you with "cross site collection"-lookups, but in case you can change your requirement to multiple sites in the same site collection and break permissions instead, it will solve your problem.
There are lot of solutions which can solve cross-site Lookup, including third party tools and code. But cross-site collection lookup solution is strange to me. I googled out this blog tough, maybe it useful to you.http://sharepointsolutions.blogspot.com/2006/11/free-custom-cross-site-lookup-column.html
You will need custom code for this, but since the amount is minimal and this is Stack Overflow I assume that won't be a problem. You will want to make your own listview web part that queries the list using ListViewByQuery. An example can be found here: http://blogs.msdn.com/b/ramg/archive/2009/04/21/implementing-a-simple-cross-site-collection-list-view-webpart.aspx