I used to have a rather monotonous voice as well; I still do, when I "lecture" (either officially, or when I get started on one of my obsessions). However, after I got the habit of listening to books on tape, I began to use the same inflections and expression I heard in the narrators' voices. My speech today is formal; but it has the right inflections in it, and is understandable. I have also recently begun copying colloquial speech, with moderate success.
The key here is to learn the rules of the tone and speed of speech, and to follow them; some languages use tone and speed as part of the words themselves (tonal languages, ex. Chinese), and Aspies in those countries must learn to speak just as we do in our non-tonal languages. I think acting or speech classes might also help.
I would think of inflection in speech as having the same function as punctuation in writing. When you ask a question, your pitch goes up at the question mark. When you recite a list, you pause at commas. When you say something important or exciting, your voice becomes slightly louder and higher pitched, as with an exclamation point. At the end of a sentence, your pitch goes down where you would put a period. Think of it as "verbal punctuation"--and imagine how tough it would be to understand a book without punctuation marks!
You may never have perfect inflection--I don't, but then I'm only 23 and have more to learn--but you will be more understandable. Learning inflection is a worthwhile effort.
[EDIT: Oh, I forgot--Also a part of inflection are the stressed and unstressed syllables of words. You will see these marked in dictionaries with apostrophe-like symbols or boldface type; they are rather subtle when spoken and must not be over-done, but their presence is essential to proper pronunciation of words.]