Aight, I've tried testing this out, and I've been asking around for a definate answer, all to no avail.

Basically, I need to know what makes a unit decide to attack 1 over another. Here's what I've got so far:
1) If one unit has the ability to attack it, it will attack that unit first. (A guardian will hit a rine before it hits an SCV)
2) If it starts attacking a unit, it will finish it's attack on that unit unless it either:
a-----gets attacked by a unit (If the first unit can't attack it)
b-----The unit becomes un-attackable (Invincibility, cloaking w/o vision or detectors, whatever.)
c-----The unit is rendered unable to attack, due to death, immobilization (Maelstrom or lockdown, something like that)

Now, That's it. I need to know what makes it pick from 1 rine to another. I really hope to god it isn't random.

Units target based on priority. If it's not useful, then it wont target it. So in the case of two marines, with equal value, they will probably target the same marine.

They also have an urge to attack players differently. In most games, it seems that Player 3 will be attacked more than Player 2. Use that to your advantage?

I did make a map once that relied on the fact that guardians (alone) will attack the closest building (they were all owned by the same player and were of the same race) as long as that building didn't have the ability to attack. Sorry I can't be more specific but I'd bet that the definition could be expanded to many more situations. Just needs testing.

Well, since noone else is responding, I would like to mention one thing that I've noticed (though I've not bothered testing it) :
Units tend to de-prioritize hero targets. This can be seen in, say, Temple Siege, where a spell that summons lots of Dark Templar won't work against enemy heroes when other, normal targets are sufficiently close to be automatically engaged.

Can anyone else give any useful information?

If the unit can attack back, guardians will usually attack the closest one.
If not, it'll attack it until an enemy comes along that can return fire.
so basically what you said...

Gar, I need more concrete information. Does anyone have any really useful information?!??!! I like the tries, but is there anyone out there with a "key" or something that tells you the rules at which units attack by and choose their targets?

cpu units attack based on unit scores. if you put a tank and a rine next to each other, the guardian would attack the tank.
im sure you can find a list somewhere on this site, i remember seeing a unit score list.

What about same unit scores?

whichever is closer in that case

No, not always. Are you just guessing?

I need to know this 100%. There are times when the unit will attack the furthest 1 away.

i have absolutely never seen a cpu unit attacking the one farther away, its always the one in front that gets owned. which makes it extremely easy to micro against cpus

I believe melee units don't change target if they are chasing a unit (not buildings) until they give up the chase.

Lockdown puts a unit below everything except non-attacking structures.

That's about all I can tell you