I've considered the JD/MBA route, but from what I understand, conventional wisdom says that it's probably not worth it unless your in a T14 school. Generally speaking, a JD is sufficient if you want to practice law and an MBA is enough if you want a business career. There may be a few careers where a dual degree is desirable, but unless you know for sure that you will need both, it's just another year of paying tuition and not working. Probably not a good ROI.
However, if you decide a JD/MBA is right for you, Miami is a respectable program for both business and law and they seem to be generous with scholarship offers. Forbes magazine ranked the MBA program #65 in 2003 and they are #73 for the law program (USNEWS). The median GMAT (2003) was 650. I assume, as with most schools, work experience is not a strict requirement, but most people there will have at least a few years. Most schools take a few students directly out of UG as long as you can show leadership potential.