Fring, the free IM app, lets you make Skype calls. However it will not work over 3G unless you get the patched version, which would require a jailbreak.
I finally got Skype (through Fring) to work reliably on my iPhone using wifi. Of course, now that I have something like 1200 rollover minutes saved it hardly seems like I need it. But you never know.
Jailbreaking is basicly a hack that allows you to do things Apple wont let you. You can install Cydia, where lots of free jailbreak apps are available, and you're able to SSH onto the phone and modify files and such.
So with a jailbroken phone you can install non-official App Store apps, such as patched versions of apps.