FIRST and foremost – IF you have previous knowledge of the other website then YES I would consider it unethical to register the similar name. Moreover Id say youre not very creative in that it wouldnt be very original.
With that said this is done ALL the time – I have had it done to me half a dozen times or more. At the start of this year I registered a domain with an s and later found out the non s name was already in use. Example slotsfreak slotfreak – I had grabbed the former thinking slots sounds better than slot 
did I feel guilty – yes, but hell by the time I found out I already had the site designed and up.
Most instances of this are minor and most coexist in a friendly – possibly unknowing manner. However, its good to do some research prior to registering a domain name – you alway want to take the time to register the dash versions and possibly the .net version as well.
Randy said it best with his remarks – would it create any possible confusion in the marketplace – if the answer to that question is yes then you should almost always stay away.