For gaining root on N900 you need rootsh installed which is again from the repositories, which are currently down. I bet that's also the issue you are having, that maemo repositories are down: http://talk.maemo.org/showthread.php?p=1314588
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http://talk.maemo.org/showthread.php?t=88707

For using sudo you need to have the package sudser. Best would be to download the rootsh.deb manually from one of the mirrors and install it. It will allow you to further investigate and play with apt-get