Good news! Following on from the weekends release of an iPhone application for SeaDragon MS has released an experimental silverlight viewer for Photosynth. So if you have a Mac, just install the Silverlight plugin and try it here.
It doesn't seem to have all of the functionality of the full client, and clearly won't benefit from DirectX acceleration but it does a pretty good job I reckon. I've been trying it on work's Macbook and it's nice and speedy. Try it out and see what you think.