It took a lot of fiddling and googling but finally I got PXE booting to work using my DHCP scope on IOS. So, here's the code:
ip dhcp pool ITS
network 192.168.1.0 255.255.255.0
bootfile bootpxeboot.com
next-server 192.168.1.3
I have other things in the scope but these are the important parts. Why am I doing this? I want to get Windows 7 on my old HP TC1100. No optical drive so network boot is about the only way to make it happen. I used these instructions and tweaked them a little to use the IOS DHCP scope. Looks like it's loading just fine now. Slow via tftp but it's working!