You know, I have been a regular visitor to the Straight Dope site for several months now, and whenever I drop by, I usually check out the threadspotting section. But this is the first topic that got me interested enough to actually register so I could join in.
As a mathematician by education, programmer by occupation, and linguist by hobby, I am forced to agree with those who say math is discovered and the language of math is invented. (I have only read the first page of this thread, so I’m sorry if I repeat information contained in the second page.)
The ancient Egyptians, the ancient Chinese, and the ancient inhabitants of India all discovered certain properties of right angles, certain properties of prime numbers, and pi. (Admittedly their values for pi were not as precise as ours.) Right angles, prime numbers, and (ideal) circles may all exist only as abstract concepts, but when three very different and physically separated cultures all come up with the same abstract concepts and work out the same properties for working with those concepts, it makes me think that those abtract concepts have a reality independent of the specific minds that first thought them, especially when those concepts become useful in modern celestial mechanics, which the those minds had no concept of at all.
Perhaps that reality exists only within the human mind (i.e., the human mind in general, not the mind of a specific human), but in that case mathematics has existed since the first human mind came into being, and is discovered, not invented. If these things are products of how humans perceive the world, then they are hardwired into our minds and they are discovered as properties of the human mind rather than as properties of nature, but they are still discovered. I personally do not believe that they are properties of the mind only, but the only way to settle the question (short of the heavens opening up and deity revealing the answer) would be to meet an intelligent alien species that perceived things in a different way and had a system analagous to our mathematics that was nonsensical to us but actually worked.
Another example of independent discovery of mathematical principles would be Newton and Leibniz with calculus. But I suspect that some on this list would say they were simply discovering unknown properties of a system that had previously been invented.
As further support for the idea that mathematics exists as a property of the human mind in general (again, I do not believe this is the case, but I believe it is possible), Noam Chomsky postulated that all humans have the innate ability to learn language. This part of our mind is termed a language acquisition device, or LAD. While most of Chomskyan linguistics is no longer widely accepted, the concept of an LAD is still considered valid by most linguists (with the exception of behavioralists). Now, If mathematics is only an abtract construct, then it must exist in all human minds (due to the example I provided above), and just as we use the LAD to learn a language, we use an analagous ability to learn the language of math. Language helps us to express thoughts, the language of math helps us to express mathematical concepts.
However, I believe that mathematics is as real as gravity or mass; that is, it is a property of the physical world.