I think tanks can see you, but only when you're very very very close. If you're not moving, nobody but the aforementioned units can see you. In fact, it's my favorite stealth tactic: Spot infantry or a vehicle, stop moving, wait for it to go by, and then shoot it in the back of the head

I agree with Crazy Ivan though, the Stealth Black Hand should have a one-shot-one-kill weapon that works only if aimed at the back of the head. It'd be quite rewarding... And make those camping Havocs paranoid
As a counterpart, Patch should get one too (I don't know, most people tend to ignore patch, and every time I attempt to sneak in with GDI and use Patch, my attempts are successful).

Btw Ivan, nobody uses the cloak pack anymore