According to Gigaom.com, the announcement may be made at the CTIA Wireless event taking place in Las Vegas next Wednesday.
The P2P voice and IM service has been downloaded more than 405 million one billion times, and with the iPhone being so popular, it's only natural that the two be paired together. A standalone Skype client might make a lot of Skype-fans happy. And for Skype to stay on top, the move to mobile is the way to go.
We'll have to wait until next week to see if the rumors are true.
[Update March 30, 2009]: So, it looks like the rumors were right. Skype for iPhone will be available Tuesday, March 31. You obvioulsy still can't use Skype on your iPhone with 3G, but it will work fine with Wi-Fi.