Can anyone give some insight on the likliness of getting a spot off a waitlist of a school you have been accepted to?
There are no generic answers to this question. It is one that can only be addressed by the admissions office at the specific school in question.
Some schools use the waitlist as polite denial. Others use it for its intended purpose.
Schools that lose a lot of students to "summer shrinkage" (i.e. have a lot of depositors withdraw) tend to go to their lists.
One thing to keep in mind. Wait lists are used to generate full-pay students to fill in when admitted students do not fill up every last slot in the freshman class. Generally, the less financial aid you appear to need, the higher up the list you may be pushed (this does not hold true at the truly elite schools - think Ivy League - where full need is met without loans). So if you have a ROTC scholarship to a school where you are on the wait list and do not decline it (not going to talk ethics here), you may want to let admissions know that you do not need FA as a condition of acceptance. It may help.
The best questions you can probably ask admissions are:
1) Where are you on the waitlist?
2) How far down the waitlist did they get the past 3 or so years?
3) (After May 1) Did you fill all your slots through your initial offers - and how many slots are left if you didn't?
Of course, let them know of any change in your financial aid status.
One last thing to keep in mind about being a waitlist student - you will likely be at the bottom of your academic class, unless it is a highly selective school where every admit is a crap shoot. If you are in over your head academically, keeping good grades while participating in ROTC will be a challenge.