well, you didnt way your distrobution, but there is usually a file called rc.local in /etc/rc.d, all you have to do is add those lines to it, and it will run on boot. you may want to give a timeout factor to the dhcp client. you will have to man your dhcp client though. this way, it doesnt sit there and sit there for a minute or so when you boot. just set it for 10 seconds or so, then if that doesnt work, you can get one manually. with dhcpcd it is the -t option, so on my system i have
dhcpcd -d -t 10 eth1
the -d option just makes it print my mac and ip address if it gets one. i dont know how it is with dhclient, but im sure there are plenty of options that will do the same thing. but if there isnt a timeout option, somewhere somehow, i would suggest getting dhcpcd. cause rc.local will run and complete before you can login, so you dont want to be siting there if you take you laptop somewhere away from the AP, or into another network where you need different options. good luck.
if there isnt an rc.local, post back, we'll figure it out. make sure to give your distro though.