use vars qw($spaces);
my $spaces = " " x shift;
while (<>)
{
    s/\t/$spaces/g; # Tabs to spaces.
                    # Perform substitution the other way 
                    # for spaces to tabs.
    print;
}
#
[download]

It's not one line, though. In golf mode: perl -pe 's/\t/q( )x$n/ge;' replacing $n with some number.

There's a program somewhere in textutils, in the GNU toolkit, that will do this too.

Update: Changed according to MeowChow's suggestion.