Is it possible to get an online MBA through a "top tier" b-school in the USA? I suppose it depends on how you define "top tier."
BusinessWeek has a list of top 30 full-time business schools. Within that list, Indiana University (Kelley) is the only school that offers a true online program. Carnegie Mellon (Tepper) is also on that list of top 30, but the online program they offer is very selective and you have to live within specific cities to participate in that program.
Thunderbird is considered a "second tier" school by BusinessWeek. They offer an online MBA. Arizona State (Carey) and Babson are other "second tier" schools that offer an online MBA degree.
There are many other schools that are ranked on BusinessWeek (either for their Executive MBA or their part-time MBA). Some of them also offer online programs. Schools that come to mind include: Drexel (LeBow); the University of Florida (Warrington); Villanova; U Mass Amherst (Isenberg); and several others.
I find that the phrase "online MBA" or "distance learning MBA" can be leading because some programs still require a considerable time on campus. Therefore, it's important to clarify and to differentiate which programs are 100% online. I often meet physicians and medical students who are interested in online MBA programs. Most of them are interested in the didactic learning and less motivated by the social networking aspects of b-school. I suppose it really depends on your reasons for pursuing an MBA and your overall career goals.