jnunemaker, maybe this post will help?
http://groups.google.com/group/datamapper/browse_thread/thread/b03963bf05a7ae60/b9d71f1f8e9dd020?lnk=gst&q=nslinkmodule#b9d71f1f8e9dd020
It's a different gem, but seems to be a similar cause...
There's another solution:
Mysql NSLinkModule error
// If you get an error like this:
dyld: NSLinkModule() error
dyld: Library not loaded: /usr/local/mysql/lib/mysql/libmysqlclient.
15.dylib
Referenced from: /usr/local/lib/ruby/gems/1.8/gems/mysql-2.7/lib/
mysql.bundle
Reason: image not found
Trace/BPT trap
// You solve it like this:
sudo mkdir /usr/local/mysql/lib/mysql
cd /usr/local/mysql/lib/mysql
sudo ln -s /usr/local/mysql/lib/*.dylib .
Let me know if either of these helps?