LimeWire does not want you to query the Gnutella network for alternate sources in its current shape. Ignore the "search for keywords instead" red herring. First of all, if you only have the SHA-1 that's strictly impossible, thus nonsense. LimeWire only "encourages" searching by keywords because this makes it much more likely that the query terminates soon.
Wasn't this sufficiently explained in this thread:
http://groups.yahoo.com/group/the_gdf/message/22146
If not, then read this older thread:
http://groups.yahoo.com/group/the_gdf/message/20774
The sole purpose of adding a DHT to Gnutella is making searching by SHA-1 (or possibly any other unique identifier).
efficient and thus support it again. I don't think anyone intends to use a DHT for keyword-bases searches.