I'm not a lawyer, but...
Technically, you should be safe:
Copyright does not protect the idea
for a game, its name or title, or the
method or methods for playing it. Nor
does copyright protect any idea,
system, method, device, or trademark
material involved in developing,
merchandising, or playing a game. Once
a game has been made public, nothing
in the copyright law prevents others
from developing another game based on
similar principles. Copyright protects
only the particular manner of an
author’s expression in literary,
artistic, or musical form.
In practice, distributors seem to ignore this, probably in fear of legal costs, or in simple ignorance of this law:
"... I have received an email warning
that my game was suspended from
Android Market due to a violation of
the Developer Content Policy... I
emailed Google asking what is the
reason for the application removal.
Google promptly answered that The
Tetris Company, LLC notified them
under DMCA to remove various Tetris
clones from Android Market. My app was
removed together with other 35 Tetris
This kind of empty threat probably wouldn't have been as effective in the past, when games were sold not just by one or two distributors but by numerous vendors.
So if your game is very similar to another game, it's probably a bit of a gamble. You should probably be able to tell them to get lost, so long as your distributor (Apple, Google etc.) doesn't side with them.