I'm following up on this thread as I think it is not worth starting a new one. I've been using RB for quite some time now, loved it since the beginning and especially appreciated the dynamism of the development (regular, worthy updates). 
That being said, I'd like to add my $0.02 with another round of humble suggestions for AJAX RB (tested on IPhone).

First, some comments and suggestions about the ITunes-related functions. 
As Eric mentioned it, I also find playback buttons on the "now playing" screen difficult to manipulate. A possibility could be to simply grab more screen space by "getting rid" of Safari's address bar. They do it in Signal, not sure how though (seems like the AJAX page is scrolled down, the address bar is still there, just "hidden"). 
In fact, I really like the way the guys from Signal mimic the IPhone/ITouch native interface, especially the now playing screen. Having volume controls (and rating) in "poping" menus is convenient and space-efficient. With RB's current "now playing" screen, I do not have access to volume control directly and have to switch to "remote mode", and even activate itunes behavior if it isn't already. 
To sum up my "wish list" :) : 
- bigger control buttons 
- volume control 
- some kind of "poping menu" to hide/show volume and rating control? 
- and... as developed thereafter, the possibility of switching directly to the music browser

Is there a rationale behind having separate items for "Music" and "Now playing"? Both are ITunes-related, and I personally would love being able to go back and forth: 
- consider "now playing" as the ultimate item in the hierarchical menu. 
- being able to switch to "now playing" from the music browser (one big button ?) 
- switching directly to "now playing" when selecting a song 
- from "now playing", having a back button to access the music browser

Concerning the music browser itself, here are my unworthy remarks: 
- I love the "letter splitting", actually more efficient than a search functionality 
- I'd love to be able queue songs in some playlist, instead of playing them directly (some kind of "+" button in front of artists, albums and songs names?) 
- the back "<" and forward ">" items appearing aside each category/artist/album/song are, for me, redundant. When I think about it, the forward button does the same as clicking on the item. For the back button, only one is needed as it is a tree structure. For me, the perfect solution would be to have a HUGE back button at the top (e.g., in the light gray title bar). Ideally, you could add another huge button to reach directly the "now playing" screen, of course ;). I realize your rationale behind those duplicated buttons is probably related to scrolling, but I personally would not mind quickly scrolling to the top of the page to reach those shortcuts. 
- I experienced slight performance issues when browsing especially large list (like my entire album collection, as a stress test), but nothing alarming given the technical constraints. I did not dare trying with my whole track listing...

I love the trackpad mode, really handy for presentations. No particular suggestion there.

Concerning the remote control mode, I guess that "emulating" hold functions must have given you some headaches. Duplicating the buttons seems like a good solution, but it somewhat clutters the interface. My main "problem" comes from not having the "click" ones on the periphery. I personally use them more than hold ones, maybe it's just me. Also, slightly bigger buttons would be more comfortable. A shortcut/button displaying the current behavior and allowing to switch it could be great too. There is some vertical screen space to be taken (again, the address bar), but I admit I have not found the ultimate layout yet... I realize it's a tricky problem and that you visibly already gave it a lot of thought. I'll let you know if I have any proposal.

Again, thanks for the amazing work towards making RB (and now, my IPhone) the ultimate companion of my computer. All the updates/surprises I had in the past year are way more than worth my initial investment. I'm really looking forward to the next steps!

Best regards, 
-- 
David