Is Lucene/CLucene used currently in used in current implementation of KDE Desktop Search? That is Nepomuk/Soprano? Is Virtuoso is meant as a replacement/competing library to Lucene? as were Sesame and Redland?

Best optimized for its task - retrieving and storing index like information. That what the desktop search is about. Well established in both open source to enterprise It's list of usage is so impressive:
http://wiki.apache.org/lucene-java/PoweredBy

Virtuoso seems to feature nice commercial support from OpenLink, nice set of features, even rather enterprise scale then useful for usual desktop searching. Does not seem to be so spread as Lucene.

Does Virtuoso always run an SQL database? It seems quite heavy for desktop.

Once upon time there was CLucene and Strigi and thing looked so promising, just the GUI needed improvements. But then came KDE & Nepomuk and I feel like stepping back. Complex of technologies, constant changes in backends, lousy frontends, no standalone UI only KIO well lagging behind kio-beagle or kio-lucene. Kerry was discontinued. Kickoff does not do file search anymore since I upgraded to KDE4

I hope things will get better, in meantime I stick up with Beagle and Tracker, the both now lacks KDE integration, but at least they do search well.

As far as I am aware, clucene/lucene is not currently in use by KDE.
Nepomuk provides services on top of an RDF data store, which it accesses in an abstract manner through Soprano. Soprano has several backends, currently Redland, Sesame2 and Virtuoso.

Nepomuk uses Strigi to index the files on your system, and stores the extracted metadata and text in the currently used Soprano backend. For more detailed information on how it all works, I recommend you contact it's developers.

Please note however, that the user interface, particularly in Dolphin has improved in KDE 4.6.

There are several parts of KDE that depend on clucene, but I don't know what its role is.

clucene is required by soprano and strigi, according to my package manager (pacman, archlinux)

CLucene is likely used only for data retrieval, Strigi as Nepomuk uses it just passes data along to Soprano I believe.