I believe Romney is mistaken in this case. While 47% of the people do not pay any federal income taxes, it is wrong to presume that his lower tax message will not appeal to any of them. Some are young and just do not earn enough yet, but aspire to earn more and really are somewhat conservative. Others understand that thriving business means employment and want to move into the 53% who are paying taxes. Others are simply rational and understand that if spending bankrupts the country their entitlements cease.
I'd estimate that at least 20% of that 47% are potential Romney voters, and could be up to 40% and should not be written off so easily. Do not forget that some taxpayers, like actors and news reporters, are part of the 53%.