If you're willing to (or allowed to) switch to typewriter (monospaced) font, you won't encounter the problem: the regular and bold fonts have glyphs of the exact same widths.
The following MWE uses Courier (scaled 5% in order to equate the cap-heights of the mono and text fonts in use), because in that font the difference between medium weight and bold is particularly strong. The lmodern package has a bold mono font as well, but the difference to the medium weight isn't as pronounced.
\documentclass{article}
\usepackage[scaled=1.05]{couriers} %%
\begin{document}
\noindent
\texttt{The quick brown fox 012456789}\\
\texttt{\bfseries The quick brown fox 012456789}
\end{document}
Another font package possibility you may want to consider is arev. (Given that you seem to be preparing for a beamer presentation, using the arev package would be a good choice anyway.) In the arev package, the mono font harmonizes well with the text font (which is a sans-serif font), and the medium and bold mono fonts are quite different.
beameroverlays? --e.g.\textBF<1>{text}– Li Wang Jul 21 '11 at 10:06\only<1>{\textBF{text}}or put this definition into\def\textBFitself. – Jul 21 '11 at 10:28\newcommand<>\textBF[1]{\only#2{\sbox\CBox...}}, where the dots mean all the rest in Herbert's definition. – egreg Jul 21 '11 at 10:33\onlyhere will lead to a different effect in my situation --- i.e. thetextwill disappear in other slides. – Li Wang Jul 21 '11 at 10:36\only<1->so that it appears from slide 1 up to the end. Right? – Jul 21 '11 at 10:40\only. How could I make\only<3>{\textbf{text}}works like\textbf<3>{text}? – Li Wang Jul 21 '11 at 10:48\newsavebox\CBox \newcommand<>*\textBF[1]{\sbox\CBox{#1}\resizebox{\wd\CBox}{\ht\CBox}{\textbf#2{#1}}}works for me. There was some funny invisible unicode character in the code of my last comment. It might have caused that (Herbert: Feel free to add this code to your answer) – Martin Scharrer Jul 21 '11 at 11:07