Serials on SecurID tokens are not secret information. The seed that is used to calculated the keys is not related to the serial, and keeping it secret will not enable anyone to crack the authentication codes more easily.
There is one exception: RSA itself keeps a record of all the seeds and which serials use them, and if that record is breached all bets are off. This has happened once before. In this case, you're screwed, and you have to replace all your tokens with ones whose seeds are still secret. Hiding the serial might protect you for a few hours longer than the next guy, but the token codes themselves can also be used to find which seed from a list matches your token.
At the end of the day, SecurID isn't any more or less secure than the common TOTP codes generated by things like Google Authenticator. It's the same principle, and if Google loses those seeds, the same thing happens.