Indeed, this is an issue.
I fly with a spot PLB on longer cross countries and over remote terrain. I think its great for safety, and my girlfriend can also keep tabs if she is concerned.
Since I installed my garmin 430W and GTX345, I have noticed a very occasional MSG on my screen. Usually before I can get my hand to the button, its gone and everthing works fine.
One day I caught it on camera and saw this
by Caleb Lesher
, on Flickr
I have since figured out that it only happens when my SPOT is in the "breadcrumb" tracking mode and placed on the glare shield. I still use the SPOT, but it seems to not interfere if I keep the device farther away from my GPS antenna