In CRM 4, this used to be a very popular request:

"Please change the default lookup type on the customer field to contact"

Also…

"Please limit the lookup type on the customer field to only allow selection of contacts"

In CRM 2011, there still doesn't seem to be an easy way of doing this, so we have to revert to 'unsupported' means:

document.getElementById("customerid").setAttribute("lookuptypes", "2");
document.getElementById("customerid").setAttribute("defaulttype", "2");

If I find a more supported way, I'll update this post.