Gopalan Nadathur and his team have developed the Teyjus implementation of λProlog. Version 2 was released in April 2008. Its compiler is written in OCaml and it now supports separate computation, more effective uses of types at run-time, a restriction of unification to the higher-order pattern fragment, etc. If you are interested in the current practice and state of λProlog, please lookup Teyjus.
The FAQ is still preliminary, but a good place to start. There is a brief overview of documentation on λProlog as well as a bibliography that includes papers on the theory, design, applications, and implementation of λProlog from between 1985 and 2000. Dale Miller and Gopalan Nadathur are planning to finish in 2010 a book titled Programming with Higher-order Logic. See the current table of contents. If you are interested in proofreading an early draft, please let the authors know.
The ALP Newletter (March 2010) has a general overview article about Teyjus and λProlog.
Various examples of λProlog code have been collected together into a small but growing collection. More updated examples can be found in the Teyjus distribution.
Gopalan Nadathur maintains the lprolog mailinglist: visit https://wwws.cs.umn.edu/mm-cs/listinfo/lprolog to subscribe and to learn how to post. While this mailing list is mainly intended for the discussion of papers and systems concerning λProlog and related systems, annoucements of conferences and workshops are also frequently posted there.