Async code isn't "likely" to require callbacks; it will almost certainly involve callbacks, those being a central design feature of async code. You should play with the idea for a bit before forming an opinion about it.
I don't know what you mean by "broken exception handling". This may be a Rails-ism I'm unfamiliar with. I'm very familiar with Rails (we ship a fairly large product built on it), but I've never tried to shoehorn async I/O into it; like Twitter and presumably every other large site, we do Rails on the front-end/UI and fast things on the backend, in our case with EventMachine.
Like I said: I'm not arguing that Rails threading is bad, or even that Rails should have better async support. If I cared that much about the performance of my front end, I probably wouldn't be serving requests directly off Rails. Rails developers may very well be better off with threads. But that fact has little to do with the merits of threading and async as concepts.
Take the following Python-esque (but not Python) psuedo-code in a threaded language:
header = read(socket, header_size)
log("lost socket while processing headers in packet")
This is one of the reasons I've spent the last few years fleeing event-based code towards things like Erlang, rather than running towards it; I've been doing event-based code for non-trivial work and things well beyond "demos" and the plumbing just explodes in your face if you want to build actually-robust software where simply crashing in the middle of a request isn't acceptable. Despite my best application of good coding practices and refactoring you still can't get close to the simplicity of something like Erlang code.
(By the way, if you are stuck in evented land, one of the things I have learned the hard way is that anywhere your evented API has an error callback, you absolutely must provide one that does something sensible. I now always wrap such APIs in another layer of API that does nothing but crash as soon as possible if no error callback is provided. If you can't figure out what your error callback for a given such call should be, that's your design trying to tell you something's wrong.)
That said, your (admittedly hypothetical) example is hideously broken. In EventMachine, and written properly:
@buf << buf
rescue => e
The mistake you made (and it's common to a lot of evented code) is in driving the entire system off raw events. Don't do that. Buffer, to create natural functional decomposition.
Evented code is rarely as simple as synchronous code, but there's no reason it has to be needlessly choppy.
That said, I think this design is overvalued. Yes, it's true, you (probably) can't always wrap an entire request's processing in a single exception handler in any evented Ruby library I know about. But I wouldn't wrap an entire request handler in a single exception handler in any case! If I was preparing to deal with a database exception, I'd wrap the code making the database call in an handler for the database exception. If I was preparing for a filesystem exception, &c &c &c.
Incidentally, I've been doing very, very, very large scale evented systems for going on about 10 years now (large scale: every connection traversing tier 1 ISP backbones), and, sorry, this stuff has never blown up on me. I may have been shielded from exception drama by working in C/C++, where exceptions are not the norm. I was a thread guy before that. Threads definitely did blow up on me, a lot.
Under the hood, everything's event-based (with optional preemptive multitasking), there's just varying levels of compiler optimization that affects how much you have to do manually and how much you have to worry about it. The inner event loop of Erlang and the inner event loop of Node.js and in fact the inner event loop of just about anything nowadays looks pretty much the same.
That's not the way in which I say evented code blows up. If you can write like that, it doesn't blow up, because you don't have to sit there and basically manually implement your own stack handling if you want anything like that sort sane exception handling, it all just works.
Since this is a terminology issue there is, as always, grey areas, but since I mostly use the term evented in the context of the Node.js hype I tend to use it that way. I've been doing stuff like your snippet for a while too and it hasn't blown up on me either, which is why I'm so down on the style of coding Node.js entails, which does.
The point of my snippet is not that that is a brilliant choice intrinsically, the point is that you don't have the choice and end up implementing anything like that manually.
something = some_io_that_needs_an_event_callback()