Hi I have some problem too with WP Multi Network and Wp 3.1 … I have modified the code for the menus, I can create new network, but wen I move a site(assign site) from a network to anther the slug isn’t correct 
If someone as an advice …
Hi,
I had the same problem I have commented from line 174 to line 199 in the
registration/register.php (in child theme)
Here the code
<div id="blog-details"class="show">
<span class="label">
<input type="checkbox" name="signup_with_blog" id="signup_with_blog" value="1" checked="checked" />
</span>
<label for="signup_blog_url">
<input type="text" name="signup_blog_url" id="signup_blog_url" value="" />
<input type="text" name="signup_blog_url" id="signup_blog_url" value="" />
</label>
<label for="signup_blog_title">
<input type="text" name="signup_blog_title" id="signup_blog_title" value="" />
</label>
<label>
<input type="radio" name="signup_blog_privacy" id="signup_blog_privacy_public" value="public" checked="checked" />
</label>
<label>
<input type="radio" name="signup_blog_privacy" id="signup_blog_privacy_private" value="private" checked="checked" />
</label>
</div>
