When writing scientific text I normally use the siunitx package to typeset numbers (using \num{}) and numbers with units (e.g. \SI{4.0}{TB}) in LaTeX.
Something that bothered me for a while was the fact that when making presentations in Beamer, numbers and units would be typeset in a serif font, whereas beamer uses sans-serif fonts.
The solution is actually documented in the manual, but what to me a while to figure out was that in Ubuntu 12.04 (and therefore in Debian as well, I presume) the version of the siunitx package is lagging compared to what is available on CTAN. Ubuntu 12.04 still has version 1.3a of the siunitx package, which was released in September 2009. For this version the solution is to add the following to the preamble of your document: