dyld: Library not loaded: /usr/lib/libsqlite3.dylib
Referenced from: /System/Library/Frameworks/Security.framework/Versions/A/Security
Reason: Incompatible library version: Security requires version 9.0.0 or later, but libsqlite3.dylib provides version 1.0.0

Web searches produced a lot of information including some workarounds that didn’t. Finally I read that Selenium-RC 1.x has some hardcoded paths in it that mean it won’t work with Snow Leopard, and you need to get the Selenium-RC 2.x jar (alpha at this time). The easiest way to do this is to install the selenium-rc gem that Pivotal packaged.

[Update]

More recently, have been having issues running Cucumber/Webrat/Selenium on new MacOS systems. It turns out that Webrat contains an out-of-date version of the selenium server jar file. If you are using Webrat, you need to copy the good selenium-server.jar from the selenium-rc package over the version contained in the Webrat gem. Something like:

 
sudo cp /Library/Ruby/Gems/1.8/gems/selenium-rc-2.2.1/vendor/selenium-server.jar /Library/Ruby/Gems/1.8/gems/webrat-0.7.0/vendor/selenium-server.jar