The ifup script has an 'if bridge then exit' where my patch had 
'if bridge and in_hotplug then exit'. 

So what was supposed to prevent a recursive invocation of ifup from being
triggered by the 'brctl addbr' line, in fact prevents it from working at all :)

The (soon to be) attached patch fixes this, cleans up the error handling in the
case where either brctl or the kernel support are absent, and causes radvd to
reload its config, since the MAC address of the bridge may change on addition or
removal of devices.