To take the authentication with Sinatra a bit farther you may want to use another service to do your authentication against. This is some sample code adapted from a Rails example. This code takes use of the linkedin gem from Wynn Netherland to do the authentication and also make some calls to the LinkedIn API. There are other gems that just do authentication for many services like Facebook and Twitter but for this sample I wanted to be able to make additional calls to the LinkedIn API.
If you know of any way to make this code better please comment or fork the Gist.