Provided you're not talking about one or two letters misspelling difference and provided you're not ripping off another sites design/colour scheme, I'd say you have nothing to be worried about.
I know there is law somewhere stating that you're not allowed to register and operate a website that is a deliberate misspelling of another domain and you can get sued and lose ownership of your domain if caught doing so, especially if you're building a similar site to the one you're using a misspelling of.
Say for example you register; faecbook.com or faceebook.com and then start a social networking site, you're pretty much going to lose any court case Facebook brings against you regardless of stature or who has the most money.
It's why a lot of counterfeiters for physical goods get caught and shut down because they're imitating a product and always mostly using a deliberate misspelling.
I would advise personally against registering and building a brand around a deliberately misspelled domain name, it's not worth the time.
Just come up with something yourself, there are many many domain extensions you can creatively use to come up with a great domain name. I personally use: http://domai.nr/, which generates some unique domains and shows if they're available or not.
