Makes sense to me.

(The only way i can think of that it might work, is if the is SomeClass attribute of a parameter indicates that an exception should be thrown if something other then SomeClass is passed in -- but that seems like a non-standard use of the "is" keyword)

That's pretty much how it works (as I understand it), but it's only a side-effect of the fact that you're giving the parameter a type. That is, it's standard in the same way that:

my SomeClass $foo;                # a.k.a.  my $foo is SomeClass;
my SomeOtherClass $bar = stuff();
$foo = $bar;                      # compile-time exception
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producing an exception is standard when SomeOtherClass is not a child of SomeClass. It just has the benefit of letting us make the compiler enforce some of the "correctness" of the passed values.

Again, this is just how I understand it from the Exegesis and Apocolypse. I could be horribly wrong, of course. :)

Update: Re: The sub parameter being an alias. Am I correct in remembering that one of the Apocolypses indicated that the properties (such as type; or it may have just been the type) are properties of the name we give the variables, and not the actual value/container? This is why the fact that the parameter is just an alias doesn't bother me.

bbfu
Black flowers blossom
Fearless on my breath