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