From everything anyone (including lawyers) have told me, you can get sued for anything. The real question is whether or not you will, and whether or not they'll win.
One lawyer told me, "if anyone thinks you're big enough to take a run at, then that means you're successful." In other words, suing someone is a big undertaking. It has to be worth someone's time and money.
That doesn't mean you won't get a strongly worded cease and desist letter.
(Of course, IANAL).
Assuming you didn't directly copy any code or screenshots or whatever, then you're probably not breaking copyright rules. Patents, on the other hand, are overly broad, and if someone big enough to sue doesn't like you, they likely have a patent that covers anything you might be writing.
Further reading: it would be worth looking at the "look and feel" part of this case.