I like the design (though perhaps a bit gradient-heavy, I prefer flat) but you should definitely add some sort of tagline, or at least put the "what is zipalong" link in a different color or something; I didn't even see it the first time I opened the site. Just seeing a login form, I immediately closed the tab. Good thing I read the comments, because this looks really cool.

One small usability tip: you could pretty easily combine the login and registration forms. Just put an extra "register" button, do a username check, then add a second password box and the name fields. Also, it'd be a little nicer (though I see what you're doing with the in-one-line thing) if you stacked the password confirmation box under the other; I usually visually compare the lengths before hitting "enter".

Also, I noticed that the look and feel is in many ways similar to Flickr, especially the colors. I don't know if that's a positive or a negative; on the one hand, it will immediately associate your site with photography for anyone who's used Flickr before, while on the other hand it may make it harder to establish a unique brand.

I haven't explored the inside of the site much, but from what I can tell, you've done a pretty good job. Keep up the good work, and good luck.