Using Terminal Services Gateway.
One of my users was working on TS on and off yesterday, but in the evening they started getting:
"This computer can't connect to the remote computer because the Terminal Services Gateway server's certificate is expired or revoked. Contact your network administrator for assistance."
The workstation is running Vista (no Service Pack). No updates made on the Server or the Client in period started failing. All other workstations using TS Gateway work fine.
Most recent change was install of CA issued cert to replace self issued cert.