<p>Getting the subdomain from a URL sounds easy at first.</p>

<pre><code>http://www.domain.example
</code></pre>

<p>Scan for the first period then return whatever came after the "http://" ...</p>

<p>Then you remember</p>

<pre><code>http://super.duper.domain.example
</code></pre>

<p>Oh. So then you think, okay, find the last period, go back a word and get everything before! </p>

<p>Then you remember</p>

<pre><code>http://super.duper.domain.co.uk
</code></pre>

<p>And you're back to square one. Anyone have any great ideas besides storing a list of all TLDs?</p>

Anyone have any great ideas besides
  storing a list of all TLDs?
No, because each TLD differs on what counts as a subdomain, second level domain, etc.
Keep in mind that there are top level domains, second level domains, and subdomains.  Technically speaking, everything except the TLD is a subdomain.
In the domain.com.uk example, "domain" is a subdomain, "com" is a second level domain, and "uk" is the TLD.
So the question remains more complex than at first blush, and it depends on how each TLD is managed.  You'll need a database of all the TLDs that include their particular partitioning, and what counts as a second level domain and a subdomain.  There aren't too many TLDs, though, so the list is reasonably manageable, but collecting all that information isn't trivial.  There may already be such a list available.
Looks like
http://publicsuffix.org/
is one such list—all the common suffixes (.com, .co.uk, etc) in a list suitable for searching.  It still won't be easy to parse it, but at least you don't have to maintain the list.
A "public suffix" is one under which
  Internet users can directly register
  names. Some examples of public
  suffixes are ".com", ".co.uk" and
  "pvt.k12.wy.us". The Public Suffix
  List is a list of all known public
  suffixes.
The Public Suffix List is an
  initiative of the Mozilla Foundation.
  It is available for use in any
  software, but was originally created
  to meet the needs of browser
  manufacturers. It allows browsers to,
  for example:
Avoid privacy-damaging "supercookies" being set for
  high-level domain name suffixes
Highlight the most important part of a domain name in the user
  interface
Accurately sort history entries by site
Looking through the list
, you can see it's not a trivial problem.  I think a list is the only correct way to accomplish this...