I've phrased my question in multiple ways but in the end, I'm really
asking the same thing. I just thought if I phrased it in a couple of
ways, it may clarify things better. Here goes:

Is there any significance or meaning to a square shape versus an oval
or round shape in the context of a universal symbol (both in US and
international)?
Is a square vs a circle suggestive of anything compared to other
symbols in use in the environment?
Are you aware of any other symbol design conventions that would
support using a round shape vs a square shape to indicate where to
"tap" a credit card on a terminal, for example?
Could using a square be confusing because of what else is used out there?

For example, we know that a circle shape is reminiscent of a target so
a circle better indicates where to "tap" your card on a terminal
pad...Any other support as to why a circle would be a better indicator
for card placement???

My research uncovered three symbolic systems incorporating a square
and a circle.  The first is the International Trail Marking System,
which is used at ski resorts.  A green circle denotes an easy run, and
a blue square denotes an intermediate run.

The second symbolic system is common throughout the
consumer-electronics industry.  A circle denotes record, and a square
denotes stop.

Squares and circles also have meanings as occult symbols.

The notion of "recording" might be applicable to your use of a circle
to denote card placement if you are recording data to the card or
recording data from the card into the terminal.

Sincerely,

Wonko

"The International Trail Marking System uses: 
Green Circle - Easiest Runs 
Blue Square - Intermediate Runs 
Black Diamond - Difficult Runs 
Double Black Diamond - Very difficult Runs"

"Trail Maps, Signs & Terms On the Pistes" MaxLifestyle.net (2005)
http://www.abc-of-skiing.com/pistemarking.asp

"Sony TC-WE305 Dual Cassette Deck" Musicians Friend Inc. (2005)
http://www.musiciansfriend.com/products/240693.jsp (click on the
Zoomable Image link to view the button symbols)

http://rapidshare.de/files/1663454/Consumer_Electronics.zip.html (more
images of circles and squares in the consumer-electronics context)

"CIRCLE (sacred hoop, ring): An ancient and universal symbol of unity,
wholeness, infinity, the goddess, and female power. To earth-centered
religions throughout history as well as to many contemporary pagans,
it represents the feminine spirit or force, the cosmos or a
spiritualized Mother Earth, and a sacred space. (See next item)
Gnostic traditions linked the unbroken circle to the "world serpent"
forming a circle as it eats its own tail."

"SQUARE: In contrast to the circle which often symbolizes the sacred
and spiritual (including the "sacred" earth), the square represents
the physical world. Like the quartered circle, it points pagans to the
four compass directions: north, east, south and west. While the circle
and "spiral" symbolize female sexuality in many earth-centered
cultures, the square represents male qualities."

"Symbols and Their Meaning" http://www.radioliberty.com/Symbolsandtheirmeaning.html

Check out this website on Gestalt Theories of Visual Perception:


http://www.csusm.edu/iits/trc/training/lessons/graphicDesign/d2Perception/2_design_perception.htm

Here is an excerpt:
Strong Figures Are Stable

Although everything in the perceptual field does influence everything
else, it is also true that some figures are more resistant to
contextual influences than others. Figures that tend to resist such
influences called strong figures or ?good gestalts? generally share
the properties of simplicity, regularity, and symmetry. In addition,
such figures tend to be ?closed,? that is, they are surrounded by a
continuous, unbroken contour, a property called ?closure.? For
example, a circle, because it is simple, regular, symmetrical, and
closed, is a very strong figure. Squares, rectangles, and triangles
(which are also simple, regular, symmetrical, and closed) are also
considered strong figures. However, these figures are not as simple as
the circle (because they have corners). Neither are they as
symmetrical (circles are symmetrical with respect to any axis but
squares, rectangles, and triangles are not).

Strong figures resist change or disintegration under poor viewing
conditions or variations in the viewer?s attention. Further, strong
figures tend to win out in competition with weaker figures.

Very interesting stuff. I'll keep this page bookmarked. But it hasn't
really answered my question(s).

Please keep searching.

Thanks,
Renee

I figure that people are most likely to tap on what they can best see,
independent of whether they equate that specific shape with a
particular application, hoping that it will do something.

Personally I don't equate either shape with an application except to
say that the automatic tellers I use all have buttons that are square
or rectangular to look like a keyboard.

Good Luck