Thanks, the de-select code is what I was after.
And for what it's worth, the constant 0x000F used above is simply the logical OR of this:
LVIS_SELECTED | LVIS_FOCUSED | LVIS_CUT | LVIS_DROPHILITED
and as LVIS_CUT is defined as "The item is marked for a cut-and-paste operation.", unless your implementation requires it (and mine did not), it can be omitted.
In my case I was also using ModifyExtendedStyle(LVS_EX_CHECKBOXES,LVS_EX_CHECKBOXES) which puts a checkbox next to the text label in the first column. And it has the peculiar behavior that you can click the label which will perform the normal selection behavior (LVIS_SELECTED | LVIS_FOCUSED | LVIS_DROPHILITED), but if you click just the check box, it does not. That is, if you select any text in Row 1 it'll be highlighted as you'd expect. But if you then toggle the check box in Row 2, Row 1 remains selected as you handle the LVN_ITEMCHANGED message for Row 2. And in my implementation, I wanted to emulate the standard Select / Multi-select behavior (using GetKeyState(VK_SHIFT) and GetKeyState(VK_CONTROL)) in addition to the check box. And I should probably note that if you select the text label next to the check box, hitting the space bar toggles the check box, an artifact from long ago.