Low If Crowd is sitting on example.com, you are not able to set the cookie domain to .example.com. This should be allowed, see http://stackoverflow.com/questions/1062963/how-do-browser-cookie-domains-work
Relevant code is in UpdateGeneral.java:
if (!(domain.equals("") || domain.equals(actualDomain) || (domain.startsWith(".") && actualDomain.endsWith(domain))))
{
addFieldError("domain", getText("options.domain.invalid"));
}