In general, I would tell employers about that sort of thing and ask for a quick two-line email that they have no objection to $NAME_PROJECT_HERE.
P.S. The right time to do this was before starting work on the app, since your options are very limited if they say "Cool, we own that now, any questions?"
If you don't have a contract, then you have bigger problems anyway.
As other people have pointed out, you could release your app under a different company - but then you have a similar situation of technically working for two employers. Again, that shouldn't be a problem unless there is a potential for conflict of interest.
For copyrighted works: In the U.S., anything done "within the scope of employment" (which can be a fact-intensive question) is considered to be a "work made for hire," with the employer as the "author" and thus as the owner.
stackoverflow.com/questions/401269/states-having-nonassignable-invention-exceptions-similar-to-california-code-sect
I don't know what the default is in cases where there's no explicit agreement, but this outcome seems possible.
www.dilbert.com/2009-01-22/
Forgive me that the links aren't clickable, I'm having trouble submitting this comment and I suspect it's a bozo filter that I'm tripping.
But as long as I disclose my side projects they are mine and the company fully supports us in our side projects.
My advice: Read your employee contract and then talk to HR - make sure everything is legit before you make your move. That way there will be no surprises.
(Developed while not employed) || (Developed in (California||Delaware||Illinois||Minnesota||North Carolina||Washington) && Developed on your own time using your own hardware) || (Your employment agreement/contract specifically assigns you IP rights)
If yes, then yes. If no, then no.