To beat a dead horse...
I thought I remembered a version by Walter using cumsum, but I couldn't find the link. Here's my attempt to recreate:
n = 5;
m = 3;
a = zeros(m,n);
a(1,:) = 1;
b = cumsum(reshape(a,1,numel(a)));
Running Daniel's test, I got ~0.65 seconds and ~1.05 seconds for the reshape&cumsum and repmat&reshape versions, respectively. However, what you save in compute time is eaten up by doing extra typing.