Hm. Turning off eth0 is pretty easy. If your using Slack 10, then you need to edit your /etc/rc.d/rc.inet1.conf file and either remove the eth0 information or just comment out the lines for eth0. In earlier versions of Slack (and maybe 10 depending on how you set it up) that info is in the /etc/rc.d/rc.inet1 file.
As for why ath0 isn't working with dhcp, that could be more difficult. If eth0 is functional at the same time as you are trying to get ath0 working, it could just be that your computer is confused as to what interface to use. Your ping results kind of suggest this. Another thing to try is to turn off your eth0 with ifconfig eth0 down and then re-do your nmap.
when I do "dhcpd -d ath0". It just sits there scanning
Just as a check, the proper command is dhcpcd
not dhcpd. If this isn't a typo in your post, that could be the problem.