SolidWorks 2010 seems to handle split surfaces in an arbitrary way. Some operations remove them, other keep them. Here's a list that I've figured out so far. Feel free to edit.
Mutual Trim - Removes Split Lines
Singe Trim - Does not remove Split Lines
Thicken - Removes Split Lines
Knit 2010 - Does not remove Split Lines (except from planes)
Knit 2009 - Removes Split Lines
Message was edited by: Samuel Schweighart