Recently I've seen a lot of Polymorph macros based on the construction:

/cast [harm] Polymorph
/stopmacro [noharm]
/p Sheeping %t! DO NOT TOUCH!

I have a couple questions about how this works, but no one on the UI forums seems able to answer them so I thought I'd bring them here.

1. How successful is the [harm] [noharm] pairing at eliminating false warnings? Does it revent warnings when the target is (a) a same-faction player or npc, (b) a member of an un-sheepable race, (c) otherwise immune to Polymorph (boss?), (already crowd-controlled), (e) already sheeped, (f) out of range, (g) sheeped by someone else between the time i started casting and the time my spell hits, (h) manages to resist my spell?

2. If [noharm] alone is capable of detecting a sheeped target, would "/cast [harm] Sinister Strike" prevent me from breaking sheep? If so, which other crowd control methods would it preserve? Seduce? Freezing Trap? Sap? Kidney Shot?

There is a release 2.0 macro thread you should be posting this in (a quick Search should turn it up fairly easily).

Or check the WoW UI & Macros forum. Specifically:
"harm - Evaluates 'true' if target is hostile"

So the answer to 1 is that only (a) is true.
The answer to 2 is no and none.

So much for that. Thanks for the quick answer, Oaken.