我认为此版本的mongoDb客户端缺少一些默认配置.尝试运行:

I think there is some default config what is missing in this version of mongoDb client. Try to run:

mongo 127.0.0.1:27017

这很奇怪，但是后来我感觉到问题消失了:)(所以简单的命令"mongo"没有任何参数对我又开始起作用了)

It's strange, but then I've experienced the issue went away :) (so the simple command 'mongo' w/o any params started to work again for me)

[Ubuntu Linux 11.10 x64 / MongoDB 2.0.1]