I've worked at places that tend to keep their hardware around pretty long. Also, many windows shops still run 32 bit OSes on their 64 bit hardware, I don't see the 32 bit problem disappearing as fast as10gen assumed.
Also netbook atom processors are still 32 bit. Mongo has potential for a datastore for desktop apps as an alternative to MS Access and SQLite. Finally, most smart phones have 32 bit CPUs, and there is potential for using mongo on some smart phones.