I don't know if the assumptions that were made in this study requested by Kerry and Dorgan are correct - absent an actual Bush plan for social security it's impossible to know what assumptions are correct - but I have yet to see a single argument which actually tries to genuinely refute the idea that social security private accounts would be an incredible burden on small businesses.
Right now the onus is on business to pay their social security taxes on time. They're credited to the worker when they're supposed to be paid and applied to the retirement benefit formula accordingly. If businesses are a bit late in paying taxes they'll pay a penalty but it isn't a big deal. But that's because benefits under the current system are formula based and not return based. If your employer doesn't pay your benefits for a couple of months, and the stock market shoots up 20% in that time, are they liable? If not, why not?