I think in most cases I wouldn't jump with excitement when a free service would start providing a paid version charging $25 per month. But Trello is such an amazingly valuable service that it took me just five minutes to get company credit card in my hands and upgrade our organization to Trello Business Class. We have learned recently, most notably through experience of Google Reader, that services without a commercial support cannot survive regardless how many devoted users they might have. Therefore I was very happy to see that Trello has found a solid business model that will keep the service running and improving in the future. Trello Business Class doesn't provide many additional features over standard version that is free of charge yet. So far they offer only synchronization with Google Apps accounts, having outsiders as observers on boards, bulk data export, and more granular access control. I'm pretty sure the developers of Trello will make wise decisions what to put in the free version and what to charge for. So far they have done an amazing work at navigating Trello towards being the best project management tool ever invented!
It seems the Business Class option is so fresh that Trello folks haven't managed to write a single public page about it yet. I'll provide a link with more information once available.