$\int f(x) dx $ is appearing as $\int dx f(x)$. Why? A few of us over on MITx have noticed that $\int f(x)  dx $ is appearing as $\int dx  f(x)$.
It's not the maths of it that worries me. It's just I recently read a justification (analytical?) of the second form somewhere but can't recall it or where I saw it.
Can anyone give me a reference?

The second form sometimes makes it easier for the reader to match variables of integrations with their limits. Compare 
$$
\int_0^1\int_{-\infty}^{\infty}\int_{-\eta}^{\eta}\int_{0}^{|t|} \Big\{\text{some long and  complicated formula here}\Big\}\,ds\,dt\,d\zeta\,d\eta
$$
and
$$
\int_0^1 d\eta\int_{-\infty}^{\infty}d\zeta\int_{-\eta}^{\eta}dt\int_{0}^{|t|} ds\,\Big\{\text{some long and  complicated formula here}\Big\}
$$
