Invalidation would be as you describe...prior art or in this case obviousness negating the patent...but it doesn't even have to go that far...usually most patents have 1 or 2 claims that are the "meat" of the patent...what gives it it's power of enforcemnt...so lose any of those and invalidating that particularr claim(s)...and yes you could have a revalidated patent but if it's missing that all important claim or two....IT"S IMPOTENT