I'm a lefty, too, and just can't cope with the idea of putting a messenger bag on the "wrong" shoulder. (though the USAT Timbuk2 bag had me seriously tempted)

Thinkin' a backpack is in my future just for the bus commute I'm doing. Maybe one of those Ortlieb systems that let you turn a pannier into a backpack.

Keep us posted on how riding with a "righty" messenger bag goes, ok?