I have minimum knowledge of Clojure, so is there some motivation for single-pass other than perhaps simplicity?
Edit: A brief outline of the tradeoffs: http://en.wikipedia.org/wiki/Compiler#One-pass_versus_multi-...
Clojure, like many Lisps before it, does not have a strong notion of a compilation unit. Lisps were designed to receive a set of interactions/forms via a REPL, not to compile files/modules/programs etc. This means you can build up a Lisp program interactively in very small pieces, switching between namespaces as you go, etc. It is a very valuable part of the Lisp programming experience. It implies that you can stream fragments of Lisp programs as small as a single form over sockets, and have them be compiled and evaluated as they arrive. It implies that you can define a macro and immediately have the compiler incorporate it in the compilation of the next form, or evaluate some small section of an otherwise broken file. Etc, etc. That "joke from the 1980's" still has legs, and can enable things large-unit/multi-unit compilers cannot. FWIW, Clojure's compiler is two-pass, but the units are tiny (top-level forms).
What Yegge is really asking for is multi-unit (and larger unit) compilation for circular reference, whereby one unit can refer to another, and vice versa, and the compilation of both units will leave hanging some references that can only be resolved after consideration of the other, and tying things together in a subsequent 'pass'. What would constitute such a unit in Clojure? Should Clojure start requiring files and defining semantics for them? (it does not now)
Forward reference need not require multi-pass nor compilation units. Common Lisp allows references to undeclared and undefined things, and generates runtime errors should they not be defined by then. Clojure could have taken the same approach. The tradeoffs with that are as follows:
1) less help at compilation time
2) interning clashes
While #1 is arguably the fundamental dynamic language tradeoff, there is no doubt that this checking is convenient and useful. Clojure supports 'declare' so you are not forced to define your functions in any particular order.
#2 is the devil in the details. Clojure, like Common Lisp, is designed to be compiled, and does not in general look things up by name at runtime. (You can of course design fast languages that look things up, as do good Smalltalk implementations, but remember these languages focus on dealing with dictionary-carrying objects, Lisps do not). So, both Clojure and CL reify names into things whose addresses can be bound in the compiled code (symbols for CL, vars for Clojure). These reified things are 'interned', such that any reference to the same name refers to the same object, and thus compilation can proceed referring to things whose values are not yet defined.
But, what should happen here, when the compiler has never before seen bar?
(defn foo  (bar))
(defun foo () (bar))
In Clojure, the form doesn't compile, you get a message, and no var is interned for bar. You require other-namespace and continue.
I vastly prefer this experience, and so made these tradeoffs. Many other benefits came about from using a non-interning reader, and interning only on definition/declaration. I'm not inclined to give them up, nor the benefits mentioned earlier, in order to support circular reference.
CL itself has two different compilation interfaces COMPILE and COMPILE-FILE.
A file compiler may implement a multipass strategy, where calls to later in that file provided functions are resolved.
A compiler may also provide different kinds of compilation units. IIRC CMUCL does that. See "block compilation".
What is the rationale behind not letting 'declare' declare vars in other namespaces? I've run into that when dealing with circular dependencies, and sometimes it seems that would help.
And the complexity/utility tradeoffs must be considered.
fwiw, you get a warning at compile time regarding the undefined symbol. And in the AllegroCL IDE, for example, uninterning is just a matter of hitting [Return] when you get the error message (dialog). I believe uninterning the conflicting symbol is a common restart in other implementations as well.
So, how is this compilation unit different from all other compilation units?
For example, how would you do dataflow analysis at compile-time (which is necessary for a plethora of optimizations) with just macros?