Looks like YouMail has gotten to the bottom of the issue. Turns out that it was a bug in the code (a bug they were aware of, no less) and they didn't realize how severe the issue actually was. They've since corrected the issue (or are in the process of), and will be back in the Android Market shortly. For full details, check out the YouMail Blog
YouMail, a popular (and free) visual voice mail solution, mysteriously disappeared from the Android Market today.