We've just upgraded to JIRA 5.1 and I was expecting to see a button next to status on an issue that pops up the workflow diagram, but it's not there. I've rummaged in the manual and can't see any reference to this on the pages relating to workflow diagrams.
You need to have the View (Read-Only) Workflow permission.
https://confluence.atlassian.com/display/JIRA/Managing+Project+Permissions
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