You should be able to use @reboot in place of the first five fields - just like most BSD systems. Well, at least you can in 10.1 - I haven't verified it in 10.2. (For some reason, the crontab(5) manpage doesn't appear to be on my Jaguar system, so I can't be sure.)
Of course, the OS X-ish way is to create detailed scripts in /Library/StartupItems that will be scheduled and run in the proper order by /sbin/SystemStarter, according to which services they require and provide. Generally, these are turned on and off by setting flags in /etc/hostconfig. See /System/Library/StartupItems for Apple-supplied scripts.