I think there is little risk of Trello being discontinued/FogCreek going out of business. They do a lot of hiring and have had a pretty vocal customer segment pleading for them to charge for it. I don't really understand why they haven't tho I'm sure Joel Spolsky has a plan.

Disclaimer: I wish he was charging--I have a similar project I've been working on over nights and weekends. It is very hard to compete with free and difficult to position/market a product you have created to appeal to everyone. That being said--I love working on my project so it'll be around for a long time... Link: http://www.thetaboard.com