You can test the new Photoshop Express online app here:
Adobe Photoshop Express

Here is a review, which mentions some important points about the terms of service:
Review: Adobe Photoshop Express | Webware : Cool Web apps for everyone

I tried PX in Firefox on Ubuntu 7.10, and the demo ran very well. Only when loading images off the web into the demo gallery was there any huge delay for me. Most of the edits are basic, and processed very quickly. It's interesting, at least.