I was reading my diploma out loud today and I realized my attempts thereat are hampered by the fact that I don’t know how to pronounce the number 2003 in Latin. Help please?
Would it be “duo mille et tres”?
Better than my answer. I was going to say “Emm emm eye eye eye”. 
The cardinal number is duo milia [et] tres (“two thousands [and] three”). The et it optional. However, if there’s an A.D. or Anno Domini in there it gets complicated. I’m not absolutely certain, but I think that you use the ordinal number with A.D. (“two thousand and third year of the Lord”). In that case it would be tres et bis millesimus in the nominative case. You want it in the ablative case to agree with anno, so it would be tribus et bis millesimo. But don’t quote me, because it’s been almost 18 years since I saw the inside of a Latin classroom.
If you want to know how the ancient Romans would have pronounced the letters M and I, you’re looking at the wrong guy. Once while in my college library, I saw on the shelf an entire scholarly book dedicated to the of the names of the letters of the alphabet in Latin, and controversies pertaining thereto. But I didn’t read it. I may be a geek, but there are limits.
I don’t think mille declines. Only use plural or singular. Of course, I don’t have a lick of Ecclesiastical Latin so I could be wrong, and will even accept that I am likely to be wrong.
Speaking of pronunciation. Do we know how Latin was pronounced? Similar to Italian? I once asked a Latin teacher if she knew how it might have been pronounced back in the day but she wasn’t sure. She spoke it in a very monotone manner.