Flickr has launched a new iPhone-optimized version, and it looks fantastic. The most used options run along the top of the page and, although there doesn't appear to be any AJAX-y cacheing going on, the individual pages load at a fair clip.
You can do anything of the viewing you can do on the normal Flickr, although the bite-sized version doesn't offer any editing of pictures -- the Organizr, for instance, isn't there. It also comes with small, slimline and faster essential search and viewing tools.
The slideshows are taken off, and you'll need a standalone iPhone application like Fraser Speirs' Exposure (iTunes Link).