The short response
No. Evolution is a theory in a very specific technical sense, but it is not speculative.
The longer response
I. Evolution is not speculative
When creationists dismiss evolution as “just a theory,” they mean by this that it is at best speculative and at worst a mere guess. However, the scientific consensus recognizes that evolution is confirmed by evidence as strongly as anything can be. It is with good reason that evolution is considered the cornerstone of modern biology: as Theodosius Dobzhansky famously and rightfully pointed out, “nothing in biology makes sense except in the light of evolution” (Dobzhansky, 1973).
II. In what sense is evolution a theory?
If evolution is the cornerstone of modern biology, why do scientists sometimes still use the word theory when talking about evolution? Doesn’t this prove that they themselves think evolution is speculative? On the contrary, scientists use the word differently than the average person on the street. In scientific circles, the word theory usually denotes a wide explanatory framework that has reached the highest standard of confirmation by evidence. When scientists want to talk about a mere conjecture that has yet to be confirmed, they use the word hypothesis. As zoologist Tim Berra explains, “[a] scientific theory is the endpoint of the scientific method, often the foundation of an entire field of knowledge, and is not to be confused with the sort of ‘theory’ we so easily propose in everyday conversation” (Berra 1990:4).
To say that evolution is “just a theory” is at best to make a mistake comparable to saying that Copernican theory, electromagnetic theory, quantum theory, the theory of relativity, the theory of continental drift, and the germ theory of disease, to name a few, are just guesses. Evolution is a theory only in the same robust scientific sense as all of these other theories. Creationism, on the other hand, is not even a theory.
III. A creationist organization disavows the assertion
To its credit, the creationist organization Creation Ministries International advises its followers not to assert that evolution is just a theory (CMI n.d.). The assertion CMI recommends instead (namely, that evolution is an unsubstantiated conjecture) is equally false, but at least does not try to exploit confusion about scientific terminology.
Berra TM. 1990. Evolution and the Myth of Creationism. Stanford: Stanford University Press.
CMI. n.d. Arguments we think creationists should NOT use. creation.com/arguments-we-think-creationists-should-not-use. Spotted 21 Nov 2014.
Dobzhansky T. 1973. Nothing in biology makes sense except in the light of evolution. The American Biology Teacher 35(3):125-129