Dear Florian,
So far, OpenVPN has been working very well for me. Unfortunately, the iPhone doesn't have (yet?) an OpenVPN client, so I'm forced to work with what's available.
The options are: L2TP, PPTP and IPSec. If you were to install a VPN endpoint on CentOS, which protocol would you prefer? The condition is to avoid shabby VPN servers that make the system less secure. I've seen some PPTP servers for Linux in the past but I was not impressed with their security track record. I'm not necessarily talking about crypto, I'm talking about the way the application is written.
You can set up a Linux box with Poptop (which is definitely the best solution but maybe a choice), racoon (to use with the Cisco ipsec client on the iphone) or openswan in combination with L2TP.
Best Regards Marcus