"Boadilla" is a quite plain theme in beamer class; I like plain styles, but I also want a headline in the top indicating the sections so that I can know where I am when I'm giving a speech. Thus I gave some slight changes to this plain theme: the default headline has been replaced with "infolines", while the definition of the outer theme "infolines" has also been modified as I don't have any subsections.
The original headline:
\ifbeamer@secheader\else\setbeamertemplate{headline}[default]\fi
The modified version:
\ifbeamer@secheader\else\setbeamertemplate{headline}[infolines]\fi
[notice type=download]Downdload the file here[/notice]
You may visit http://yihui.name/en/2007/10/jokes-in-statistics-a-talk-to-be-given-in-cueb/ to see the effects of my modifications.