I knew silver coins were put in the drinking skins of Roman soldiers to prevent illness. I knew it was anti-bacterial in this form. I knew they made surgical instruments out of silver too for anti-bacterial purposes. I did not know it was anti-viral in colloidial form.
Silver has a reputation, an association with purity and goodness. The only thing that disturbs me about colloidal silver use is those horror stories about people "turning grey" from overuse of silver.
If I were to use it, it would be very, very conservatively, though I'm sure those horror stories are mostly not much to worry about. I don't want to look like a space alien! :)
They also made surgical thread out of silver for the same reasons, @steveitt. Unfortunately the condition of argyria that you are referring to is irreversible, but thankfully it is only associated with ingesting large quantities of poor quality colloidal silver (particle size > 35 nm; usually homemade varieties), and even then the condition is extremely rare ( less than 200 cases reported in about 150 years).