How do I get numbered entries in a beamer bibliography

Edited answer

^{This works independently of the bibliography style, and with biblatex as well.}

According to the beamer documentation, subsection 10.6, you can use

\setbeamertemplate{bibliography item}{\insertbiblabel}

instead of

\setbeamertemplate{bibliography item}{}

so that the bib label is inserted for each bib item.

The result is:

enter image description here

^{This works in the particular case of the OP, because using the bibliography style abbrv, \insertbiblabel inserts [\theenumiv].}

I think the beamer class uses the counter enumiv to number the items in the thebibliography environment as the standard classes.

So, substituting the line

\setbeamertemplate{bibliography item}{}

with

\setbeamertemplate{bibliography item}{[\theenumiv]}

you should obtain what you want:

enter image description here

The accepted answer didn't work with biblatex as all reference numbers became zero. What worked was to substitute the line:

\setbeamertemplate{bibliography item}{}

in the code example with:

\setbeamertemplate{bibliography item}[text]

and that works with both bibtex and biblatex.