I'm not upset but it's annoying that this thread was running in cycles just like some threads over at the GDF. I assume that's because it's not fully understood what a DHT is or why Gnutella would benefit from one.

Anyway, one weak option is to use a Bitzi lookup:

http://ticket.bitzi.com/rdf/urn:sha1...NAE52SJUQCZO5C

Of course it knows only about a very small subset of all available files, it's a single point of failure and the information it provides is not trustworthy. So it's nice to have but no solution.

Also you cannot expect that any given SHA-1 actually has a corresponding existing file. And even if this file exists, it may not be available from any peer when you're searching for it. If all peers use a common DHT, it is at least easier to determine whether a file matching the SHA-1 is available or not.

Something that would be kind of interesting is running an Ultrapeer and collecting data from the routed traffic. After a while you're probably able to map most SHA-1s of popular and many other files to such information as filename (actually a list of), filesize and so on - depending on what information you record.