My 08 Honda v6 today just started blowing heat out of the passenger side - vent while continuing to blow AC out of the driver side and the ideas what would cause this?

 

Does it have dual-control temperature? Likely it's a problem with the air-blend doors. For each side there's a blend door driven by a little motor - both inside the console kinda alongside the driver's or passengers foot.

Motor might not be working, or else the linkage to the door itself might be broken. I don't know the 2008 very well but suspect there's a way to remove the panels on each side of the console & see the motors. Even without removing the panels, you might be able to hear the motors running if you turn on the key, don't start the engine, & turn the temperature dials up & down.