It would have very little effect. The Social Security Administration has evaluated many variations of this proposal:
http://www.ssa.gov/OACT/solvency/provisions/payrolltax.html
These are the two relatively simple ones:
The difference: since benefits are derived from taxed wage income, simply eliminating the cap would also increase the benefit for those affected. It won't increase much (due to the way benefits are calculated), but it will increase.
The first one eliminates the cap for the tax, but retains the cap for benefits. The second one eliminates the cap for both. Short version: all either proposal does is kick the can down the road a little further. It doesn't fix the underlying problem.