Kobayashi - 9:14 pm on Oct 1, 2010 (gmt 0)

Ok, actually you can do it in bulk within the web site at the campaign level. Click on a campaign, select all ad groups, click More button, select Bulk Edit, distribution to Networks, Apply. A dialog box pops up erroneously warning you you have unsaved changes but if you check the settings at the ad group level they appear to have saved.