Evolution is often decried as 'theoretical' because the term 'theory' is attached to it.
Scientists use the word 'Theory' in a very different way than other people do. It is much more similar to a mathematical 'Theorem' than it is to the traditional use of the word 'theory'.
Hopefully you remember high school geometry, but in case you don't, here is a refresher on the Pythagorean Theorem:
On any right triangle (a triangle with one 90 degree angle) the square of the hypotenuse is equal to the sum of the squares of the other two sides, and this is reflected in the formula a² + b² = c².
No matter what, each and every right triangle abides by this theorem. Every time it has been tested, it has given the correct result. The theorem would be easily falsified (proven wrong) if a single right triangle was found that did not exhibit the dimensions predicted by the theorem. Of course, this is entirely unlikely, if not impossible, so the formula has been added to the sum of human understanding and is considered a 'fact'.
While the definitions of 'Theory' and 'Theorem' are not identical, they are certainly more similar than the usage of 'theory' that Creationists like to use. A brief Wikipedia search is all that is required for an outstanding definition of 'Theory' in the scientific sense:
"The word theory, when used by scientists, refers to an explanation of reality that has been thoroughly tested so that most scientists agree on it. It can be changed if new information is found. Theory is different from a working hypothesis, which is a theory that hasn't been fully tested; that is, a hypothesis is an unproven theory."http://en.wikipedia.org/wiki/Theory
The following is a list of scientific theories which have been created and tested in exactly the same was that Evolution has. Creationists, do you call these 'just theories' too?
Cell theory, germ theory, atomic theory, kinetic theory of gases, plate tectonics, acoustic theory, theory of relativity.
Bear in mind that ALL scientific theories must be supported by all of the available evidence, must be falsifiable, and be subject to repeatable experimentation. Evolution has run the gauntlet just like every other accepted theory has, and it has passed every test.
A theory can never 'graduate' to become a Law. They are two different aspects of science and, while very similar, are used to describe different things. A 'Law' is used to describe a specific action or phenomenon, whereas a 'Theory' is really just a proven hypothesis: an explanation for observed events which is consistent with all of the available evidence. Most of the time, a 'Theory' actually consists of many different observations and proofs. In this case, Evolution is the observable fact, and the 'Theory of Evolution by Natural Selection' is what contains the best explanation for this fact.