Some good ressources are:
The Cucumber Wiki
http://github.com/aslakhellesoy/cucumber/wiki
The Cucmber Wiki contains a lot of information on how to write Gherkin specifications (resp. examples like some BDD-jargons would call them)
The RSpec Book
http://www.pragprog.com/titles/achbd/the-rspec-book
This book has some very concrete tips on how to use Cucumber, a lot is applicable for other Gherkin- or BDD-tools in general.
Book: Bridging the Communication Gap
On Amazon, Book Page
A great book by Gojko Adzic about the concepts and the process of Specification by Example
Upcoming Book: Specification by Example
The new book of Gojko Adzic containing detailed information and experiences about applying "Specification by Example" in the real world.
Hopefully to be published soon (I am currently reading a review edition).
Google Groups:
The following groups contain plenty of information about BDD and Specification by Example:
http://groups.google.com/group/cukes
http://groups.google.com/group/behaviordrivendevelopment
http://groups.google.com/group/specflow