Many threads about it already since the N900 was released. It's usually caused by downloading your email and checking for software updates (downloading all the repository data). Especially if you have extras-testing and/or extras-devel it can be a lot.

You can change the update check interval, for example:

gconftool -s -t int /apps/hildon/update-notifier/check_interval 10080

will set it to only check once a week. Or set it to some really huge number of minutes to effectively make it never check.

For email the only way to make it stop is to disable automatic email checks entirely, as far as I know.