If you haven't used Solr/MoreLikeThis; it constructs a lucene query using the terms in the document you are currently viewing and finds similar documents. I want to do the same in SP.
Yes, there's a concept of "similar" documents in FAST Search for SharePoint 2010. The functionality is exposed through the SharePoint query object model as well as the query web service. FAST Search Query Integration Overview MSDN article contains a reference to Find Similar that covers the concept in more details. Basically, you'll need to set the
SortSimilar property values when executing your query programmatically.