SASL stands for "Simple Authentication and Security Layer". SASL itself is nothing more than a list of requirements for authentication mechanisms (see [wiki:Authentication Authentication]) and protocols to be SASL-compatible. IMAP, POP3 and SMTP protocols all have support for SASL.
Many people confuse SASL with one specific SASL implementation: the Cyrus SASL library. Dovecot has its own SASL implementation which may at some point be separated from Dovecot itself to "compete" against Cyrus SASL library in server side. And it already can be:
Postfix as of version 2.3 supports Dovecot's SASL server implementation. See http://www.postfix.org/SASL_README.html or [:PostfixAndDovecotSASL] for details.
There's a [http://www.exim.org/eximwiki/AuthenticatedSmtpUsingDovecot patch for Exim] as well, which has since been added to Exim snapshots.
Hopefully more software will follow.