I have a WordPress multisite installation. The client wants an email update form on the sidebar of every blog. I want to use a widget there so I need to assign it to the sidebar. Is there any way to do that on the theme install ?
yes. preset widgets:
post your widget code here if you need specific instructions
If all the subsites have the same widget setup, you could hard code the sidebar setup in the static sidebar file(s) (usually sidebar.php) so every new site using that theme would have the widget. If the owner of the site uses the widget menu you'd need the email form available as a widget as well.
I suppose you could also take the form out of the dynamic_sidebar code so when a user activates widgets the form menu is always present (would have to be at the top or bottom of the sidebar). This would be a good option if the multisite is open to anyone to register and you don't want your users to have the ability to remove that one widget, but still want them to be able to add their own widgets.
There's also a plugin that you can setup multisite templates including widgets, basically you create a subsite and set all options, plugins, theme, widgets etc... on that subsite and create a template from it. When a new subsite is created it uses that template rather than the default WordPress installation. Users can then edit the default setup after creation.
I don't use multisite WordPress, but have hundreds of theme customers who do and wrote them a tutorial at http://www.google-adsense-templates.co.uk/massive-passive-profits-plugin-setting-up-a-sub-domain-blog-template-with-talian-05.html (includes a link to the plugin) they use it with an autoblog plugin. Ignore the specific theme references of course.