I needed to install xdg-utils to fix this:
Running "open:server" (open) task
finished populating things
Warning: Command failed: /generator-test/node_modules/grunt-open/node_modules/open/vendor/xdg-open: line 584: xdg-mime: command not found
xdg-open no method available for opening 'http://localhost:9000'
Use --force to continue.
./xdg-open: line 584: xdg-mime: command not found
xdg-open no method available for opening 'http://www.google.com'
node_modules/open/vendor/ contained xdg-open, but not xdg-mime.
The remote server I am using did not have have xdg-utils installed... not sure why xdg-mime was missing from the node_modules dir...