I just re-configured my router with its IPv6 address (which I got free from http://tunnelbroker.net/), and noticed that my Macintosh had not picked up an address yet – so I went looking for a command that would ask the IPv6 stack generate an IPv6 Router Solicitation message.
It seems that issuing the following pair of commands at a terminal prompt does the trick:

sudo ip6 -d en0
sudo ip6 -u en0

The first command shuts down IPv6 on the interface, the second brings it back up. There are probably equivalent commands using the ip6config command too, but I didn’t explore those.