If a sponsor goes down...

[virgohippy]

I think Mateusz's advice is a safe bet too.

If youre using paid content then you can have those links point to wherever you want, but if not, I've often wondered what the legality is concerning one sponsor buying out another's content rights...

I've seen one "dead" sponsor come back, but under new ownership... though, interestingly enough, they stopped having an affiliate program.