How to display numbers using comma as a thousands separator?

I think the siunitx package has this sort of facility.

\num[group-separator={,}]{1234567890}

Should give you 1,234,567,890

Also you can use this as a package option like so: \usepackage[group-separator={,}]{siunitx}

Be warned, this doesn't seem to work with older versions of siunitx

Seamus' answer involving siunitx is the Right Way^TM to do this. That said, it's not so hard to write a simple macro to pretty-print numbers.

\newcount\ppnum
\newcommand\ppnumber[1]{%
        \ppnum=#1\relax
        \ifnum\ppnum<0
                $-$%
                \ppnum=-\ppnum
        \fi
        \let\pptemp\empty
        \loop\ifnum\ppnum>999
                \count255=\ppnum
                \divide\ppnum by1000
                \count255=\numexpr \count255 - 1000*\ppnum \relax
                \edef\pptemp{,\ifnum\count255<100 0\ifnum\count255<10 0\fi\fi
                             \the\count255 \pptemp}%
        \repeat
        \the\ppnum
        \pptemp
}

The first time I wrote this, I used a token register rather than \pptemp, but that required writing \expandafter\expandafter\expandafter{\expandafter... which just seemed excessive.

Edit:
Given that I've now had to fix two bugs in my code, maybe I shouldn't have claimed that it isn't so hard to do this. =)

Using the http://www.ctan.org/pkg/numprint package with \npthousandsep{,} might be an alternative (found after writing the other answer - Murphy's Law!).

Btw: Grouping numbers into pairs of two can be done with the http://www.ctan.org/pkg/telprint package.