This is where Romney's cafeteria plan is brilliant. You do not get into the specific perks which Congress has given away to special interest groups. You simply set a limit and each taxpayer can pick and choose what deductions they want to take, but in no circumstances will it exceed the set amount of deductions.
So lets say the Salvation Army, Cancer Society, or your church has been a large part of your contributions, you will still make those deductions, but the total of your mortgage and other deductions can not exceed the limit on deductions. Eventually, I think Eric is correct because a person using their mortgage deduction is probably going to reach the limit, and therefore the donations to charitable organizations will not bring you any additional advantage. Now if they allow a typical three year carryover for unused deductions, it would be interesting. So if you donated a 5k car to your church, but your mortgage deduction was 15k, and the limit was 17k....then you could carryover 3k to next year and apply 2k, and carryover the remaining 1k for the next year. Romney is nuts on his tax proposals, but this is really a very good proposal.