Excel 2007 "Wrap Text" button won't "de-wrap" certain cells

FYI, I've had this same problem recently, when manipulating cells that had previously been merged. It seems the wrap text button doesn't work (won't toggle off) if the horizontal or vertical alignment of the text inside is set to justify (no idea why it became justified in the first place though).

Re-setting the alignment to left/centre allowed me to un-wrap the text again.

Excel supports line returns within a cell. Line returns are sometimes created from copied text.

One way to check for this is expand the height of the formula bar or the width of the column. Look if text is forced to a new line.

To add a manual line return click in the formula bar then Alt+return. To find the line return chr search for CHAR(10)

Another option is check for space characters. Sometimes externally generated content add spaces. If this is your situation a clear pattern will likely emerge when this occurs.