Development marches on for Canonical, and not a few weeks after the release of Ubuntu 12.10, the first live daily builds for 13.04 are already being made available. The ISOs will currently work on x86, x86_64, PowerPC, Power5, and OMAP4 systems.
Of course, as a pre-beta build, this is for development purposes only. Canonical recently announced that they will be dropping the Alpha releases from Ubuntu, so these will also be the only versions of Raring Ringtail until the beta is released in a few months time.
You can grab the ISOs from the daily build website.