OpenVPN works fine on OSX. To work with iPhoneOS, though, you're going to need a L2TP, PPTP, or IPsec server. Free/OpenSWAN is pretty easy to set up these days if you.
Honestly, the OpenSWAN documentation is pretty good, but you can find Gentoo instructions that are reasonably applicable to Arch (the default kernel config should have the options you need, so you can skip that part) here. Relevant OpenSWAN docs are here if you want to do L2TP (which makes interop with Windows clients a lot easier).