Keep it cool - good airflow inside your computer and around your
drives
is important.

Is there evidence to support this? My drives are hotter than usual
because
they're decoupled from the case. 40 degrees or so is hot but the the
manufacturers' max temperatures are 55 and 65

A good aluminium case will act like a heatsink to a modest degree,
however
if you have several drives mounted one atop the other, the heat from
each
one rises up to the next. Without adequate cooling, the drives
(particularly
those on top) will get toastier and that has the potential to shorten
their
lives. Most case airflows run from the bottom front to the top back,
pulling
in cool air near the ground, passing over the CPU and MB and out near
the
top. Often this airflow doesn't serve HDs well (excepting cases where
HDs
are mounted at the bottom). This problem can be exacerbated by using
wide,
flat, non-aerodynamic IDE cables which block efficient airflow. You can
swap
these for rounded cables or fold the existing cables in two twice over
and
strap them in the folded shape, or you can switch to Serial ATA.

There are many arguments about what temps are acceptable. Usually
there's a
trade-off between cooling noise and acceptable temps, but there are many
solid cases and low noise fans about. Personally I've found 40 degrees
to be
quite acceptable for a HD temp (also for a CPU temp too), although the
ambient case temperature should be much lower!

Like much else, these things will *contribute* to a healthier PC and to
a
longer component life.

I have the drives at bottom front, but on foam, not screwed in to reduce
noise.
CPU is way up in the 70s, but I've not had a system in 18 months so I'm
fine
with that.

On a hot day the Baracuda might touch 48, but it's speced for 65 so,
again, I'm
not worried.

Has anyone ever actually done a study showing hot drive fail sooner?

Not read any studies myself, but there is logic behind this. The tolerance
of any electronic and mechanical device isn't infinite. Pushing that
tolerance envelope will eventually have an impact on performance. I for one
would not wish to risk the data on a HD to find out what isn't tolerable
when simple common-sense techniques can prevent temperature extremes.
Speaking of which, 70 degrees for a CPU is hot. Are you using CPU intensive
applications? Even gamers and silent PC modders worry about 70 degrees! What
tool is giving that measurement? What CPU and what cooling are you using?