VPN endpoint for my IPhone?
the iPhone supports common VPN technologies like IPSec PPTP and L2TP
this Iphone VPN Guide might answer some of your questions about it
For a basic set up you might want to get a spare machine and loading Smoothwall or ipcop linux distributions. They have IPSec support built in, so it's quite easy to set up. This way you'll be able to experiment with some inexpensive hardware; figure out what works for you and implement it.