While we wait for Skype to appear in a future firmware revision, we'll play around with JAJAH. All that's needed is the internet browser--no homebrew necessary. Point your browser to mobile.jajah.com, and log in with your free JAJAH account. Your contacts will appear as clickable links. When you dial in, your phone will ring, allowing you to talk to your friend. If you're talking to a fellow JAJAH member, it'll be free.
To make the most out of the service, you should probably upgrade to the latest firmware, and take advantage of T-Mobile's free Hotspot service. Then, you'll be able to browse the web, play games, and make phone calls from PSP, all while grabbing your morning coffee at Starbucks.