I think this fear is overblown. You already see educational institutions and individuals paying different prices for box software compared to business users. Large and medium-sized businesses also probably wouldn't cheat the fee system. Again, look to boxed software: would a major company run pirated copies of Office 2007? It would be pretty easy for them to, but it's not a huge expense and if anyone found out than it would be extremely embarrassing. (It would be much easier to find out about businesses cheating with twitter just by having some superficial distinction like a badge or icon on the twitter site for paid users)
