(Before you get too excited, I'm not the person you replied to. ^_^ )
First, for your particular example, you could just do this:
If you need (a!b 4 5), it does get more complicated, and I've gotten a bit annoyed about that myself. Nevertheless, there's still a way (albeit a way which requires a bunch of typing to refactor into):
(!f:!e:!d:!c:a!b 4 5)
In practice, I rarely have more than three chained property accesses in C-like languages, or more than four things connected by ssyntax in Arc, so I think I'd just add one or two sets of parentheses and live with it. I won't pretend my case is typical, though. :-p
Of course, you can still use (((((a!b 4) 'c) 'd) 'e) 'f) to generate a.b(4).c.d.e.f. 
> mzscheme 372 does not compile for me
Did you know Arc 3.1 works on the latest MzScheme? 
 Actually, you might be further disappointed to know (((((a!b 4) 'c) 'd) 'e) 'f) is currently compiling to:
get here is a JS function not unlike rocketnia's ref . Its purpose is to disambiguate the Arc form (x y), which may compile to x(y), x[y] or (car (nthcdr y x)), depending on the type of x (function, array/object or cons, respectively).
This example works particularly well because the $("a") jQuery selector can be compiled from $!a. A challenge arises with more complex selectors, as in this snippet from the Find Me: Using Selectors and Events tutorial:
Not quite sure (I suspect it's a bug), but it seems like it has to do with the implementation of make-readtable (which brackets.scm uses).
Welcome to MzScheme v4.2.1 [3m], Copyright (c) 2004-2009 PLT Scheme Inc.
> (parameterize ((current-readtable #f)) (read))
x`y ; read in as two items
> (parameterize ((current-readtable (make-readtable #f))) (read))
x`y ; read in as one symbol
In fact arc3.1 even works on Racket, the new PLT Scheme. Only thing is that the command-line "racket" prints a newline after the "arc>" prompts, for some reason. But you can open as.scm with the editor DrRacket (as you could with DrScheme), set the language to be "Pretty Big", and hit Run; it will work.
For some reason, now I don't notice any issues with the "arc>" prompt in "racket" either. And I don't think I'm doing anything differently than I was before. ...I am forced to conclude that, when entering things into the REPL, I held down the return key long enough that it accepted an extra (blank) line of input. This explains the behavior exactly. Strange that I should have done this several times in a row... and how embarrassing. Oh well. At least now I can give racket a clean bill of health.
Yeah, I'm not trying to say you shouldn't use it for production use :)
They're opposing perspectives. As a user of arc I'd throw it into production. At the same time, from PG's perspective I'd want to be conservative about calling it production ready.
I suspect arc will never go out of 'alpha' no matter how mature it gets, just because PG and RTM will not enjoy having to provide support, or having to maintain compatibility.
 With some caveats: treat it as a white box, be prepared to hack on its innards, be prepared to dive into scheme and the FFI. And if you're saving state in flat files, be prepared for pain when going from 1 servers to 2.