A tank exists to initiate and disrupt throughout the team fight, NOT to initiate, shield and heal his allies once, and then blow up. and to GIVE the opposing team a reason to attack them, by way of damage or frequency of disruption.
Well good, this should be easier than I thought.
There are oodles of ways for a Tank to generate threat. They can do this by being an AoE sustained damage threat, they can do this with highly disruptive CC, they can do this with auras (whether positive or negative), They can do this by buffing their allies, they even do this through low cooldown shields/heals for both themselves and their allies (see: Ali and Taric). There's a great deal of overlap between a Support and a Tank because both of them function as a force multiplier. The only real difference is that traditional Supports are squishy force multipliers and Tanks are hard ones. In short, the job of both the Support and the Tank is to keep their team mates alive while giving the enemy team as much grief as possible.
So!
A Tank wants to generate threat. They want to give the enemy team a reason to not ignore them. This can be done with offensive Tank items like Sunfire or it can be done with wholly defensive Tank items like Locket. A low cooldown, constantly refreshing shield for yourself and an ally would be amazing on Tanks like Amumu who has little in the way of peeling power once his initiation is over. Taking a percentage of damage for your Mage or Ranger would be a great boon to someone like Leona who can take the hit like a champ.
If you wanted to use Riot's standard of primary/secondary class Cuirass would be a Tank/Support item. Soul Shroud could probably be classified as Support/Tank due to its active not offering you any protection, however it makes up for it with a VERY useful passive. Tank/Support items abound, from Locket's shield to Heart's IAS nerf aura. Shroud is meant to be a bit more supporty in the same fashion as Face of the Mountain, but make no mistake, it's still a Tank item through and through.