if i remember correctly you arent infringing copyright because it is a different original design, however if it is trademarked then it must be suitably different, and given that you are aware that it is similar you might find your breaching their trademark (if they registered it as a trademark) and would therefor be inforceable, if they choose to.
(please correct me if im wrong)
Its also fair to say that if they have persued their trademark in the past, it is likly they would should they see this design as if a later case arose the defendant might inquire as to why they didnt persue this case, however i am not a lawyer but have heard of an example of this in the past.