Here's what Google has to say about Titles and Descriptions in the SERPs:
Google's creation of sites' titles and descriptions (or "snippets") is completely automated and takes into account both the content of a page as well as references to it that appear on the web.
We use a number of different sources for this information, including descriptive information in the META tag for each page. Where this information isn't available, we may use publicly available information from DMOZ. While accurate meta descriptions can improve clickthrough, they won't impact your ranking within search results. We frequently prefer to display meta descriptions of pages (when available) because it gives users a clear idea of the URL's content. This directs them to good results faster and reduces the click-and-backtrack behavior that frustrates visitors and inflates web traffic metrics.
While we're unable to manually change titles or snippets for individual sites, we're always working to make them as relevant as possible. You can help improve the quality of the snippets displayed for your pages by providing informative meta descriptions for each page.
Apparently it's all automated, and it looks like titles can derive from the meta descriptions which is what I saw in a couple of examples, so if you have a short title or one that isn't very descriptive it appears a good meta description will give you more control.