How to add acknowledgments in the title (first slide) in a beamer presentation?

You can use some of the already existing commands to add the information; for example, you can say something like

\date{\today\\[2em]
{\footnotesize Acknowledgement: This work was performed within the
Mathematics and networks
(molten) project, which is supported by the Engineering
and Physical Sciences Research Council and the Research
Councils UK Digital Economy programme, with grant ref.
EPIxxx.}
}

Maybe not the perfect solution, but you can add additional information after the preamble and the \titlepage command.

\begin{document} 

\begin{frame} 
\titlepage 

Acknowledgement: This work was performed … 

\end{frame} 

Here's what worked for me;

To be fair, I only thought to do this because I generally work in LyX and couldn't implement @GonzaloMedina's solution--I tried inserting a footnote & LyX translated it like so:

\author{Mr. Indebted%
\thanks{All those lovely people I'm thankful for%
}, et al.}

This had the effect (bug?) of singling out the author doing the acknowledging--useful for me because the several authors are on different grants, etc.