Beacons Required?

In my case I would like to hear more about GPS muddle the frequency of some Avalanche beacons. Is this true? If so could it be possible with PLB also?

1. For a GPS, no. (I’ve tried many times with a GPS to cause interference with a searching beacon, with all currently available models, but no “success” thus far.)

2. For an emergency locator beacon:

a. For a true PLB -
I. I don't see how this is possible since the PLB will be off until after the search is completed. (Keeping the PLB on at all times during a tour might draw down the proprietary and expensive battery. I quickly skimmed the ACR and McMurdo manuals now: they caution against frequent use of the self-test feature because of this concern, and although I didn’t find any reference to premature battery depletion from unnecessarily keeping the PLB on, they also don’t seem to advocate such a practice.)
ii. For a PLB that is transmitting a distress signal, I suppose the power conversion/step-down might cause interference while searching. But I'm having a hard time thinking up a scenario in which first activating a PLB before completing the avy beacon search would be a justifiable decision.

b. For the Spot private-sector PLB-esque device:
I. Keeping the Spot on is a reasonable precaution to enhance initial signal acquisition in case an SOS message needs to be sent, plus will facilitate the sending of Okay and Custom messages. I highly doubt a Spot that is on but not transmitting will interfere with a beacon search.
ii. Ditto for a Spot that is transmitting. (And unlike a transmitting PLB, this could certainly happen during a beacon search, since the Spot might be in Tracking mode, or the Okay or Custom Message button could have been hit just before an avalanche occurred.)
iii. Later today I’ll test the above two hunches.
iv. For results with other devices: www.wildsnow.com/1609/jonathans-wildsnow-beacon-test-notes/

I've had a Garmin 60CX give ghost signals on an older Tracker 1 -- it's definitely a concern. Good idea to spread your electronics out on your body as much as possible, or turn off the non-critical ones.

I've had a Garmin 60CX give ghost signals on an older Tracker 1 -- it's definitely a concern.

Right now I took an Ortovox S1, Barryvox Pulse, BCA Tracker (most recent version), and BCA Tracker 2 (pre-release firmware). I turned all four beacons to Search, but no target beacons in the vicinity. Silence. I then turned on a Garmin 60Cx, and placed it directly on top of each beacon, numerous times. Same silence.

GPS's operate on what's known as a pseudo-single (1575+ MHz range) and for all intense and purposes it's not much different than the background noise of the universe; it ends up being turned into a position through a mathematical process called trilateration.  Avalanche beacons broadcast in 457 kHz range and the two singles do not overlap in the slightest (no chance of interference) so rest easily.  

Now cell phones on the other hand can reek havoc on your beacon and the two should never meet.

pistehors.com/news/ski/comments/digital-...ed-by-mobile-phones/

Now cell phones on the other hand can reek havoc on your beacon and the two should never meet.

As a general rule, I certainly agree.
The specifics though vary widely with the particular phone + beacon combo.  (And firmware iterations can make a big difference:  the Ortovox S1 went from among the most susceptible to among the least susceptible.)
In my testing with a CDMA phone -- and only a phone -- just having the phone on but not doing anything is perfectly okay.  (I have not been able to test with a GSM phone though.)
Making a phone call though causes varying degrees of interference in different beacons.  (And even just flipping open the clamshell causes a temporary blip -- perhaps because of the LCD screen coming on?)
"Smart" phones are an entirely different story -- very scary, even when just on, and not (apparently) doing anything.  (My testing was only for an iPhone, but I suspect that the effect would be similar among other "smart" phones.)
My conclusion is that you have a basic phone (i.e., a phone), perfectly fine to keep in on.
If you have any sort of smart phone though, turn it off at the trailhead.

Right now I took an Ortovox S1, Barryvox Pulse, BCA Tracker (most recent version), and BCA Tracker 2 (pre-release firmware).  I turned all four beacons to Search, but no target beacons in the vicinity.  Silence.  I then turned on a Garmin 60Cx, and placed it directly on top of each beacon, numerous times.  Same silence.

That's great.  Nonetheless, I had repeated ghost signals from my Tracker when I was well outside the range of a transmitting beacon. I don't know how to reconcile that with ClimberDave's post or your findings, but that's what happened.