If the field has visible borders, conditional formatting can't make the borders invisible. It can change the fill color to match the background or (by inverting the logic) make a transparent fill pattern a solid color to make an invisible field visible.
If you want field borders, you could place a second field behind it sized two pixels larger in heigh and width, postitioned to that a one pixel "border" of this field is visible behind the other. Then conditional formatting on this field can make the border appear and disappear. This may work for you if you want this field for display purposes only. If this is a field used for entering/editing data, the user can still click on or tab into the field even though they can't see it unless you take additional steps (remove field from tab order, use onObjectEnter trigger to run script to move cursor back out of the file...)
Another method, is to use a small, one row portal with invisible portal borders to display the field. Make the drop down list field part of a self join relationship for the portal and you can set this up so that there is only a visible record in this portal if "pending" is selected in the drop down.
Thanks for your response, but I am aware of most of what you have said, however, the display aspect isn't my problem...
I just can't see how to write a Conditional Formatting script which uses the result of a drop-down list field.
Click Add to add a conditional format, then select the Formula option instead of value in the conditional formatting dialog.
Click Specify and build a calculation such as:
YourTable::DropDownListfield = "Pending"
It's late evening here in the UK, so I'll give your advice a shot tomorrow.