How I just wasted 5 minutes:
Saw that Riya Beta is now open to everyone.
Went to the Riya site and signed up.
Was told that I needed Windows XP.
Realized that it wasn't open to me.
Closed browser window.
Seriously, what new web company doesn't launch with Macintosh compatibility in mind. How hard is it to write an uploader? What would it take? Two, maybe three days of a developer's time?
Or better yet, just implement a clone of the Flickr API and be done with it.
Unless the uploader is doing a bunch of preprocessing of the photos on the desktop beforehand, I can't really see any good reason for not offering a Mac client.