Question
Of all of the listed thymes, is there one that is a bad or good choice? Also can you describe the pitfalls of a thyme lawn?
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The text above is a follow-up to ...

-----Question-----
What do you think about the idea of putting Thyme in the lawn?  I'm not sure if this is possible, but I would like either to have a lawn made of Thyme or use it as a mix with the grass that is growing there already.  I'm not 100 % sure but it is probably Kentucky Bluegrass from what I was told.  What effect would thyme have on the lawn if any, and would it be noticeable?  We never put pesticides down so that's not an issue.
-----Answer-----
Thyme comes up every now and then as a proposed choice to replace the traditional American grass lawn.

For many reasons, I think Thyme would work better growing in its own area of the lawn rather than mixed in to a lawn that has a different optimal mowing height or requires certain treatments that are not favored by a Thyme home.

There are several different kinds of Thyme: Lemon, Creeping and Woolly Thyme.  You can see them at the Michigan State University Extension (http://web1.msue.msu.edu/msue/imp/modzz/00001449.html).

Lemon Thyme (http://web1.msue.msu.edu/msue/imp/modzz/00001449.html) is a low growing trailer, Thymus citriodorus.  Woolly Thyme (http://web1.msue.msu.edu/msue/imp/modzz/00001449.html)
and Creeping Thyme tp://web1.msue.msu.edu/msue/imp/modzz/00001451.html).

There is also General Info on Thyme:(http://www.gardenguides.com/herbs/thyme.htm).

Answer
Creeping Thyme (Thymus serphyllum) can take a moderate amount of foot traffic.  See the High Country Gardens website (www.highcountrygardens.com/article198.html) which posts instructions on "How to plant a thyme lawn".

Creeping Thyme is hardy in zones 7 through 9.

Can you consider Pennyroyal (Mentha pulegium)?  This semi-evergreen member of the Mint family is one of those creeping, aromatic plants that smells so good as you walk along, you don't want to leave.  Butterflies love it; Fleas and Mosquitoes hate it.  In fact, Pennyroyal is a registered Pesticide in some states.  Left to flower it will grow to 2 feet.

It needs a mild Mediterranean climate to do well, temperatures 45 to 78 degrees F.  And of course full sun.

All of these would be grown on their own, rather than mixed with lawn grass.  The weeding alone should discourage you.  But if you can succeed, Bravo.