I assume you wan to know if there are bush taxis from Zig' to Bissau. There are and they go from there to San Domingos (SD) at the border. From there you get another taxi to Bissau. Maybe there are taxis that go all the way too.
There are always security concerns on the Senegalease side, from Zig' to SD. MDC rebels etc. You should make the approriate inquiries at you embassy in Dakar, if there are any general warnings out etc. Mostly, there is no problems once your in Bissau.
You can of course avoid all this and travel to Tambacounda in Senegal, then to Kolda/Tanif and go accross the border from there. There are bush taxis etc. We did a similar thing last year, but in our own car. I have also done the Zig' to SD route a number of times, mostly without problems, but once with a problem.
Best of luck