Google's Matt Cutts posted a video explaining why and when Google may use something other than your title tag for the search results title snippet.
Matt Cutts suggested that it is best for your to try to anticipate what the user will search for when crafting your title tags. When you do that and then when it matches the query, then Google will likely show your title tag.
Google uses three criteria when determining if they should use your title tag:
(1) Something that is "relatively" short
(2) Have a good description of the page and "ideally": the site that the page is on.
(3) And that it is relevant to the query.
If you fail on these criteria, then Google may use (1) content on your page, (2) anchor text links pointing to the page and/or (3) may also use the Open Directory Project.
Here is the video:
Other title tag related stories:
- Google Not Listening To Your Title Tag? You're Not Alone
- When Google Rewrites Your Title Tags
- Google: We Show Better Titles Than Your Title Tags
- How Google Picks Your Title In Search Results
- Google Appending Brand In Front Of Search Results Title
- Google Title Tags Measured By Width Not Character?
- A Shorter Google Title Tag After Redesign? Maybe.
- Title Tags Influence Over Outbound Links
- No Title Tag? Google May Use Text From Next "Large Font"
- Google Ignoring Your Page Titles?
Forum discussion at Google+.