A Google Webmaster Help thread has JohnMu from Google explaining when Google may rewrite your title tags in the search results.

Typically, the title attribute used in your source code of your page is what Google would display in search results. But sometimes Google will change it and show something different.

John explained when Google may use the liberty to change the snippet’s title from your title tag to something else. He offered three cases, I am sure there are more:

Titles are particularly shortTitles are shared across large parts of your siteTitles appear to be mostly a collection of keywords

What do you do if this happens to your titles? John said:

Forum discussion at Google Webmaster Help.