The Fifth Amendment issue was that eminent domain should be utilized, as it states, for 'public use'. With cities going into debt, they used the arguement that the tax revenue from private commercial development became something the public would use.
Just saw a special on this on TV tonite and people are losing their homes because their neighborhoods are labeled as deteriorating, the weakest excuse possible, but accepted nonetheless in eminent domain fights, because the houses are more than 40 years old and had weeds growing nearby.