I found myself asking why this functionality was included, what would be the outcome, why it worked, and if it did work what sort of rounding was going on behind the scenes.(Also why would anyone attempt this, but that was fairly tertiary.)
As it turns out, AS3’s SHL operator can actually be used as a very fast and quite useful form of rounding. Read on for an explanation, some examples and calculated times.
var Y:Number = 6.6;
var X:int = 0;
X = Y << 8;
If we look at the integer 6, it’s stored in memory as 0x 00 00 00 06, and shifting this left 8 bits will produce 0x 00 00 06 00. That’s the expected result. All is peachy.
Now look at 6.0 stored as a floating point Number. 0x 40 C0 00 00 Shifting this left is only going to produce 0x C0 00 00 00, which is 2.0.
This lead me to believe that not only was this rounding enforced prior to shifting, but that it would be likely be handled natively. (Or at the very least, at a bytecode level, omitting the extra calls and jumps which calling a library function would incur).
Roll on an hour of programming a test bed and a year of procrastinating, and here are the results including the operations on negative values. In each case, a Number is being converted to an Integer.

As you can see, Math.floor, ceil and round behave as expected, even with the negative values. The most interesting part for me at least though is the way int() and <6.4 is rounded down to 6, but 6.6 is also.
It’s almost like it does X = Math.floor(Math.abs(Y)) * Sign(Y), but ridiculously fast.
How fast? Have a wee looksee.

That’s right. n << 0 is a good 5% faster than Int(), and about twice as fast overall as any of the math libraries.
Practical uses?
Well, obviously, it’s not always going to be the right form of rounding for every particular task, but it’s come in handy a few times, especially for example in games with positive X/Y cartesian world coordinates, or mirroring where flooring 6.4 units ahead would produce different results from flooring 6.6 units behind your character.
If anyone finds a really nice application for this, I’d love to hear about it!
Here’s the source if you’re interested. Don’t be too harsh, I wrote it before breakfast =)
http://sicklebrick.com/wpdownloads/bitwisemath.as