Ok, here's all the details I can muster up:

SVT w/ 81K miles
Today when leaving work, A/C started blowing warm. Also smelled a little musty.
Noticed a loud hissing / leaking noise. Comes from the passenger side of the engine bay.
When the A/C was off, the noise was considerably louder.
When the car is stopped, a lighter hissing noise is heard from the area circled in this picture. The hissing stops eventually.

It's not exact, but I isolated it to that area and I'm sure of what I circled.

My guess is either a leaky hose or a bad A/C compressor. Any other ideas? How much might this run me dollar wise?

gotta be a vacuum leak.

It's dark out there so I can't compare to mine, but someone here knows.

Doesn't sound like a compressor problem, just a vacuum fitting problem (which is good compared to a compressor!)

When the car is running does the compressor kick on? Do the light's dim a little? How about your vents? When you hear the hissing, and change vent settings do they move, or move really slowly?

Yes, the compressor kicks on.

The vents seem to be ok. The speed settings work as normal on the A/C. One thing I left out: On my drive home from work, when it was blowing hot, there was a "steam" or "vapor" coming out of the vents. I've seen that on cold days but not 85 degree weather.

you said you isolated the area but i think you should consider this. I have a 99 v6 and i had that same set of symptoms. look at the AC Compressor clutch while the engine is running. if it is bad it will lock and you will have a significant noise like a wine. as the rpms increase it will get louder and if you turn the ac off it is just annoying. as for the smoke yes that is how i discovered the problem turn off the ac with the engine runing and rev the engine. look where the pully is there should be a large amount of smoke surrounding it or pouring off of it. hope this helps. it cost me $250 to fix but i had it done under the counter ford charges somewhere around 5-6 hundred. im pretty sure thats your problem my friend who fixed it for me is a certified front end specialist for ford in Kansas and he said that was a very common problem with the v6s. good luck.

Yep, that's exactly what it was. Compressor totally seized up yesterday. Lots of smoke & noise, especially at start up. Will run me $600 unless I can find a better way. Any suggestions?