Virus???
I suppose it is theoretically possible if the connection to Garmin's FTP site isn't a secure one. However, someone would need to write a virus for that specific GPS unit first. I suspect that there aren't too many hackers out there that would be particularly interested in GPS viruses. It might be a great idea for wannabe terrorists though? Someone, somewhere would probably take great delight in causing a bit of havoc. I can imagine what the tabloid take would be.
I'd say it's highly unlikely.