Ultrasn0w 0.9.3 is available in Cydia stores using the repo repo666.ultrasn0w.com. To add a repo go to the Cydia store and add it as a new repository. Read instructions to learn how to add a new repo to Cydia.
The Cydia store will only be available after you have jailbroken your iPhone. If you are unable to jailbreak you iPhone yet, keep tuned in, we will post the latest details on jailbreaking your iPhone running iOS4 shortly.