<pedant mode>
And of course it wouldn't be a virus, because a virus, by definition, must be able to spread itself once it's infected a host. It would be a worm, since it wouldn't be able to be passed on from your GPS to another GPS.
Does this make a difference? Well, yes, because once Garmin notice the problem and fix it, the worm would not be able to infect any further units - the only ones which would need fixing would be those that were infected before the problem was spotted. So it would be very easy to contain.
<end pedant mode>
I've never done the software upgrade, but I'd be surprised if it caused problems... if it did, Garmin would be so busy fixing broken units they wouldn't have time to write new versions of the software!
But I have to admit, for the amount that a Garmin unit costs, I'd expect something that provided a little resilliance. A friend of mine was involved in developing the software for some of the Sky TV digital set-top boxes. They would receive updated software via satelite, without the user needing to know anything was going on. They had two sets of programmable ROM chips. Only one was in use, and the new software was downloaded into the set that wasn't being used. Once the update was completed successfully, it would switch to use the new software in the other ROM. I'm sure Garmin could include something similar. Oh well.
FFF
---------------