A minor enhancment that will allow a custom message when a site is in maintenance mode.
/modules/default/templates/admin_configEdit.html
<p>
<label for="conf[site][maintenanceMessage]">{translate(#Maintenance message#)}</label>
<input type="text" class="longText" name="conf[site][maintenanceMessage]" id="conf[site][maintenanceMessage]" value="{conf[site][maintenanceMessage]}" />
</p>
modules/default/templates/matinenace.html
<h1>{translate(#Site under maintenance#)}</h1>
{if:conf[site][maintenanceMessage]}
<p>{conf[site][maintenanceMessage]}</p>
{else:}
<p>{translate(#This site is currently under maintenance#):h}</p>
{end:}