I've now become an IPhone convict, and I was a bit sceptical about paying for applications to run on the iphone after forking out all the money for the phone in the first place, however I've become a convert with 2 new apps that I have.
Skype for iPhone - I've been looking around for several weeks for a decent way of being able to use skype with my iPhone, and was only finding suggestions of using Fring, but the poor reviews and the hassle of creating more accounts put me off. Then i stumbled across s4iphone which works by using an ajax web page, so there is a zero client foot print. The iphone simple connects to a webpage via Wifi or 3G and then you type 
It seems to poll for new messages every 10 seconds, but posts your response immediately in the tests I've performed here in the office. The product also supports voice calls, however they fake it by using your credit to call you and then linking you into a call to the other person over the net. The only downside I can see to this website is that you have to give them your skype username and password and it costs $20 after the free 10 day trial.
Roku Remote - This is a fantastic application for controlling my Roku Soundbridge which is a media streamer attached to my hifi. This application can be downloaded from the app store for a very reasonable cost of £1.78. It then allows your iphone via wifi (or over 3g if your Soundbridge is accessible via the net) to fully control your Soundbridge using the touch display of the iphone. The Roku Forums have a lot more information on this application and the some great screen shots.
Let me know if you have found any other good answers to using Skype on your iphone.