There have been tons and tons of jazz singers who accompany themselves on piano. However, I can't think of any singer/guitarists who are involved in jazz, other than George Benson (whose vocal career has largely been R&B-oriented anyway). Are there any jazz singers who play guitar and do so in a solo or small-group context? I can't think of any off the top of my head. I guess maybe Brian Setzer might fit into this category, but he's a stretch.