Nathan Weinberg reports that http://rjdohnert.blogspot.com/ received an email from Google saying you are not allowed to place the MSN Search box on a BlogSpot blog because "obstructing Google's services from operating efficiently and effectively". The blog has been removed from Google and is now at WordPress. It seems unlikely to me that this is against Google's terms of service for BlogSpot, but it is possible.
PostScript: The story is not true, Google does allow you to have an MSN Search box on a blospot.com site. More information at buzz.blogger.com/2006/03/search-policy.html.