When I worked for a major chemical company and a competitor’s US patent issued that we thought was invalid because of unconsidered prior art, we would get a “validity opinion” from an outside law firm. If that opinion agreed that the patent was invalid, we would ignore it.
Reexamination (protest it)
Another option is to request a “reexamination”. This can be done by anyone at any time. The requester sends in the prior art to the patent office and pays a fee for the reexamination. The patent office looks at the prior art and if they agree that is raises a substantial new question of validity, they then assign a new examiner to reexamine the claims of the patent. The applicant (patent owner) then negotiates with the examiner to either show that the claims are still valid, or to narrow the claims, or to even cancel all of the claims.
Declaratory Judgment (fight it)
A third option, if you think you are about to be sued by the patent owner, is to file for a “Declaratory Judgment”. This is where you challenge the patent in Federal Court.
Interference (take it over)
Finally, if a patent issues that you think you are the original inventor of, you can file an “interfering patent application” that claims the same invention. The patent office will then declare an interference, and both you and the original patent applicant will go through a process to determine who the rightful inventor is.
[edited by: engine at 3:40 pm (utc) on Mar 10, 2010] [edit reason] See WebmasterWorld TOS [/edit]