UPDATE: This MSDN posting has some interesting observations from JXJ based on his, mainly negative, experiences going down this path: http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2623565&SiteID=1

We have a business scenario where we need to link two documents libraries via a common "document ID" field.

We often use a lookup column to implements links like this.  There are several drawbacks to lookup columns, three of which are:

1. Only one column from the lookup lookup library can be linked. 
2. Performance: The source library could contain hundreds of entries.  That's too many entries in the lookup.
3. Search: There is no integrated search.  I don't mean in the MOSS sense of search, but there's no way to search / filter on multiple columns from the source document library and locate the link you want.

Backed by BDC, we can use a "business data" column type and it provides a superior search and even allows multiple columns of data to appear in list views. 

I've had some preliminary success with this approach and plan to write more about it.

If you've worked this angle before and have any comments, please share!

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