rrouter.sh : Script to reboot router
rrouter.sh [-u username] [-p password] [IPaddress]
In case you don't specify username or password, it defaults to "admin" and "admin" respectively. This behaviour can be changed by editing the first few lines of the script.
UPDATE ( 01 NOVEMBER 2010 ):
Alternatively, this seems to work on some routers but not all :
wget --http-user="admin" --password="admin" http://192.168.1.1/rebootinfo.cgi