Suppose we have a table like this:
\begin{tabular}{c}
Heading \\
1 \\
10 \\
100 \\
\end{tabular}
This will center all the numbers. If I want to align them to the right, I could change the column definition to r, but that would but the numbers on the right end of the table. What I would like is for the widest number (in this case 100) to be centered with respect to the column, and the others to be right-aligned to that number. There seem to be a bajillion packages out there that deal in one way or another with aligning numbers by their decimal points, but none seem to offer a simple way to pad numbers to a fixed width. Does anyone have a suggestion?

table-format undefined in families key. – Björn Pollex Jan 18 '11 at 11:26siunitx. – TH. Jan 18 '11 at 11:42siunitx. If you have version 1 (which sounds likely) replacetable-formatbytabformatand use\multicolumnfor the heading. – Joseph Wright Jan 18 '11 at 12:48