I've been playing around with the 20100930 build, and I'm liking what I see. It looks like you put a lot of hard work into this!
I have a few comments from playing around with it:
I'm a fan of the sidebar detent, and the way it changes based on filename length.
View > Layout - It'd be awesome if instead of using the view menu, I could just drag different views or different tabs to arbitrary locations in the window and have them snap into a grid-like formation. I feel like I'm not being very clear here, but I think this is decently common behavior, so maybe you know what I'm talking about.
Even more than the layout thing I described above, I'd like to have multi-window support (e.g., drag buffers out and back into different windows). I assume this is in the cards, but I'd like to give it an extra plug. I would use multi-window support to the exclusion of multi-pane support if it existed.
Ideally, I would be able to select multiple tabs (which already seems possible) and then drag them all out at once to make a new window, or drag them all at once into an existing window. I want to do this all the time, but I've never seen an app that lets me. Yours could be the first!
Ctrl-click to multi-select seems to work only on groups, whereas shift-click will work on either buffers or groups.
Dragging a tab to a new group in the sidebar is able to change the displayed name of the file to that of one in the destination group. Dragging the tab a second time seems to fix it in most cases.
Could the sidebar groups maybe be renamed views (e.g. "View 1" instead of "Group 1") in accordance with what they represent? I think it'd make more sense to me.
I love the new select buffer window - it looks gorgeous and matches with the rest of the UI, is super fast, and has a really great fuzzy search algorithm. I like being able to slam out letters that include part of the directory name, the filename, and the extension and have it just know what I want. It's great.
That's all I had written down. I'll let you know if I notice anything else!