The site on the iPhone works pretty good actually. I am not sure what more we would need as an app. I just bookmarked the page and added it to my home screen. Although a direct upload from Hipstamatic would be really really nice instead of having to email, etc. Hmmmm...
Oh, and if you are not using the Hipstamatic app, I very highly recommend it! I have a number of pictures posted here from that app and I love the effects.
We've got the mobile version of the site, no iPhone app for a while though
Ross