It also prints to the log upon starting a 32-bit server.

I installed it through Ubuntu (apt-get). Most people must (or should) be installing MongoDB through a package manager. Once again, assuming that people will see the warning because it's on the download page is shortsighted.

See dmaz's comment about the log files. I'm a complete mongodb noob as well, but found out pretty quickly that there's a 2GB limit on 32bit systems. It really is hard to miss.