Nokia doesn't say much about features that will be or won't be added in future updates. So it could be in the next update, but it could also happen that even the next major version of Maemo (Maemo 6) won't have it.

Unfortunately, these things are also difficult to implement as third party applications. Text to speech applications that handle more than one language are not exactly an area where GNU/Linux shines. Also, this would have to be embedded deep into the system the way S60 has it, so it can also be used to read an incoming SMS/mail/IM...