Site answering on multiple domains

Setup: WPMU 2.9.2 with BuddyPress 1.2.1
AWS hosting with Elasticache (memcached)

When we change a blog from example-site.domain.com to example-site.com, the old url still seems to work. So if you use example-site.domain.com or example-site.com, they both load the same blog in our system.

Now when converting from one domain to another we edit wp_blogs table for "domain" as well as the wp_<blog_id>_options table for 'siteurl' and 'home'. we also do some manual cache clears for certain items like site-options etc.

But it seems like wp is able to resolve both domains to the same blog regardless of the data base settings. I think we are missing a cache clear somewhere but not sure what key we should be clearing.

Essentially once the data base is updated we don't want the site to answer on the old domain and it seems like it shouldn't since technically that site/blog doesn't exist in the db anymore.

Thoughts?