Expressing capital M for molar in siunitx package

Joseph Wright just answered a similar question on comp.text.tex:

Take a look at the section 'loading additional units' in the manual. In version 1, I did include \molar and \Molar, but tightened up on sticking to SI units (almost) exclusively for version 2. The suggested definitions are

\DeclareSIUnit\molar{\mole\per\cubic\deci\metre}
\DeclareSIUnit\Molar{\textsc{m}}

According to the siunitx manual (page 14), you can create your own units easily by putting the following command in the preamble:

\DeclareSIUnit{\molar}{M}