I think we need to have SLOT='1.5' and SLOT='2.0' added to the cyrus-sasl ebuilds. But don't know enough about it to know if this is a 'bug' or not. squid wants cyrus-sasl-1.5 and postfix will accept anything... as such, if you have postfix and cyrus-sasl installed (most recent) .. then try to merge squid... you'll be forced to downgrade cyrus-sasl. I do not know enough about cyrus-sasl (damn near jack and stuff) to 'propperly' test / diagnose this.
I've build saslv2 first with slot 1, but someone told me that it isn't possible to use both sasl versions (I haven't tested it). So the best would be to build squid with saslv2. For myself I do 'USE="-sasl" emerge squid' until that has been fixed.
the ebuilds are already slotted... 1 and 2 respectively and can be installed at the same time. I'll take a look into the squid ebuild and find out if it can be changed to use saslv2
I have also added squid-2.5.1-r2.ebuild which supports sasl2 if it is installed.
