A tweek on the flat rate scnario is that it would be done in tandem with removing the raft of tax avoidance schemes (investments in Hotels, Hospitals, Section 23, rural renewals, Business Expansion Schemes etc etc).
All pay a standard rate of for example 28%. The rate would be set in order for the tax take to be fiscally neutral - in other words the same tax take as if the existing scheme in place. the result would be that there would be less of an incentive for people to be creative about their Taxable earnings, while still leaving the lower paid with the same position as at present.
I believe that this is the sort of set-up that has been adopted in a couple of the countries in Eastern Europe - Czech republic I think and one of the Balts.