S60 makes much more sense in a marketing terms because it is likely to be easier to brand in the consumer conscious. It looks like Series 60 will not vanish altogther, there are plenty of references to it remaining on the site. Indeed its likely that S60 is just a shortened form of Series 60, used informally until now, which has now become official.
This rebranding may also be the first sign that Nokia are planning to market the S60 brand more formally. Currently relatively few consumers are aware of the Series 60 designation or brand, but as more S60 phones become available Nokia may be looking to change this. While Nokia has always stressed the advantage of building to a platform from the view point of the developer little has been said about the advantages from a consumer viewpoint.
Not only would it encourage brand, or rather platform, loyalty, but it would also help clear up confusion over what applications and services could be added to the phone. All member of the value chain acknowledge that one of the biggest problems facing Series 60 is user education about what they can do with their phone. Currently, if you tell end users that can use Series 60 software on their Nokia 1234 results in a confused look. Services and applications could be branded as S60 compatiable, which would actually mean something to end users if they understood what S60 meant.
Indeed the argument that people do not realise they have bought a S60 phone or a smartphone is often used to play down the market sucess of Series with commentators dismissing the phones as souped are basic phones, or that people buy them merely because they are Nokias. A greater awareness of a platform brand, one that consumers might compare to other computer platforms, would be a good start in solving the user education problem.