I bought Joe Armstrong's new Erlang book as a beta PDF early this year and have been enjoying the material (the book is now in print). Erlang definitely has a lot of hacker mindshare but I have been unable to convince my customers to use it (so far). This may be a generalization, but those of us who love to program in Prolog are very likely to also enjoy working with Erlang. Erlang is certainly a great tool but I think it is unlikely to be very popular for two reasons: it does not provide instant gratification like Ruby on Rails and there is no large company promoting it (e.g., like Sun, IBM, etc. promote Java). That said, Erlang has a great open source community behind it and learning Erlang is very worthwhile if you occasionally need scalable applications. A comparison with Java is interesting: Java (especially with the new concurrency support in JDK 6) scales well on single servers with large numbers of cores while Erlang probably has the advantage when scaling to multiple servers.