In reply to djangopither:
> (In reply to richardhopton) Canon autofocus can fail if there is not enough contrast at the focus point.
Yep. This happens with my 350D, wearing an 18-200 Tamron.
It doesn't happen regularly or consistently, but it does happen sometimes. Try turning it off then on again. Also, shift and focus on something else the same distance away then recompose as per the original. Not ideal but works.