I cant speak for all gnutella clients (especially ones that use autoupdate features), but the safety is strongly dependant on you!!! It is just like the rest of the internet, if you dont have virus protection software and/or you dont check the files you download with a virus checker (if its not automated), then you are putting yourself at risk.

Gnutella itself is no less safe then the rest of the internet.

There is a gnutella virus out there, but it strongly depends on ignorance, that is someone would have to download the virus and run it. This virus is usually pretty obvious to spot because it matchs your search string and its a small file some times with an exe extension and sometimes with out.