The Mac comes with VPN clients for PPTP and IPSec. The former is most often used for Microsoft networks. I've used both types with pretty good success. You don't need a phone modem for this, as long as a Web-accessible gateway is present. If, on the other hand, it is a special dial-up number, then things get considerably more complicated. But jebus, man, it's 2008!
In Tiger, the VPN functionality was in Internet Connect (a separate app in the Applications folder). Now in Leopard it's within the Network pref pane. Weird, but it works well.
Cheers,
Aaron.
