Ahh yes that indeed was what I was referring too. Interestingly it is in fact (if I'm reading this right) mentioned as being exempt: "certain claims of monetary damages against the United States are exempt from sovereign immunity" [1]
I'm no lawyer, but it sounds like the Tucker act only applies to cases where the government states or implies that it will pay something and then doesn't, so the government can't just breach contracts with impunity.
[1] http://en.wikipedia.org/wiki/Sovereign_immunity_in_the_Unite...