According to the bba libor documentation the panel banks should provide rates which answer"At what rate could you borrow funds, were you to do so by asking for and then accepting inter-bank offers in a reasonable market size just prior to 11 am?"For euribor you can download the panel bank quotes. In the last days JPM provided the lowest quotes being 1bp for the maturities up to 1m. Still positive. Maybe the lowest value you can input into the reuters thompson software used to collect the quotes

? A negative fixing would mean that at least some of the panel banks say "I can borrow a reasonable amount of money from another bank, unsecured, and will even be paid interest for that". Still don't get it.Of course if the fixing is something artificial as outrun says, then it might get negative and dependent derivatives like the 3m futures can trade above 100. Which they do currently, so appearently this seems to be true. But then I doubt that the fixing is then a value in the spirit of the definition above ?