Skype says..
"We dont currently have a version of Skype that runs on Apples iPhone but we do offer Skype To Go that lets you make calls especially international calls from your iPhone (or any landline or mobile) at great rates."
I would take the fact that they say "currently" as a small hint that it may well come eventually, as they are certainly not ruling it out.
I used Fring (
http://www.fring.com/download/iphone/) on my jailbroken iPhone before I upgraded to 2.0 and it worked great, you have ICQ, MSN, Skype and basically every other IM supported as well as voice calls via Skype (also Skype out), but just via the WiFi.
But it did run in the background, so I guess either they release a none background version soon or wait till the notification service is online...
I for once will jailbreak my iPhone as soon the tools are out and get my fring back

I just hope they change that high frequency notification sound, it always made me squeeze when it rang in the middle of some very loud music in my headphone, arrgghh, just like some programs under windows which seem to have the brilliant idea to have the sound of shattering glass as error notification, you go balistic if you listen to louder music on your notebook and unsuspected that sound comes up....
Probably you will never ever see official voip apps for the 3G network...
I still have high hopes for the zattoo (
www.zattoo.com) player to come to the iPhone.
But just think about that restriction about that apps of greater then 10 MB can't be loaded via the cell network, that smells very much like the carrier do everything to keep traffic at bay.