Mark,
If the Design tool and Workflow service are on different machines, you would have to make sure you have the font installed on both machines.
Also, make sure that you are entering proper credentials for your services; that is, in Workflow go to Tools, Configure Services, uncheck "Local System Account" and put in proper credentials for the services (normally, the same domain, username and password you use to logon to the machine from Design). Restart the services.
Last but not least, make sure Workflow and Design are not installed on a "Terminal Server" (not to be confused with remote desktop connection) that accepts multiple simultaneous connections, as this is not a supported environment and could cause those issues.
Hope this helps,
Eric.