A relatively old thread at Search Engine Watch Forums discusses what the benefits of subdomains are in terms of SEO. There are many examples given, including that subdomains are often considered separate sites by Google and the other engines. That is key.
Danny Sullivan understands that subdomains are still perceived "as standalone sites and not necessarily related to a main domain."
The most important part of a subdomain is that because of the statement above, your results in the search listings won't be collapsed under the main result. You won't get those cool but pesky sitelinks, and that one sub result. You may get the first 10 results if the search term is right.
This may just be the right strategy for a company that has very bad publicity about their company name and they want to push down the bad results in Google, Yahoo, & MSN.
Forum discussion at Search Engine Watch Forums.