After making the move to Linode (finally!) I had some issues with getting the contact pages on this site and Penderry up and running. RVM was throwing an error about the sinatra gem missing. A quick scan of the error message and it was obvious that Passenger wasn't using my gemsets.
A bit of googling turned up this little snippet which, when placed in
app/config/, will tell RVM to use the gemset associated with the folder.