[Dove]

They do. The rules for classification of documents come from Executive Order 13526: http://en.wikisource.org/wiki/Executive_Order_13526#Part_1

The upshot is that when a classified document is created, it comes with a declassification date. They can be 10 to 25 years in the future. There are also specific exemptions that come with 50 or even 75 year timers. After that it's automatically declassified unless a review determines that it's still sensitive.

So you can generally expect to see declassified documents on a 50-, 25-, and 10-year delay.

If you're worried about information being classified to prevent embarrassment, though, that's illegal:

    Sec 1.7

        (a)    In no case shall information be classified, 
        continue to be maintained as classified, or fail to
        be declassified in order to:

           (1)    conceal violations of law, inefficiency, 
           or administrative error;

           (2)    prevent embarrassment to a person, 
           organization, or agency;

           (3)    restrain competition; or

           (4)    prevent or delay the release of 
           information that does not require protection in 
           the interest of the national security.

While that doesn't make it impossible that it happens, it does mean anyone with access to the information has a duty to report that sort of fraud to legal authorities if they encounter it.

[pilom]

There are 9 Exemptions to that Executive Order. These exemptions allow the government to extend the declassification window indefinitely. Number 8 in particular is very broad: (8) reveal information that would seriously impair current national security emergency preparedness plans or reveal current vulnerabilities of systems, installations, or infrastructures relating to the national security;

I don't think it would apply to this saucer as it is not a "current system" but it basically extends the time limit on any fielded system.

[lectrick]

Considering the numerous exemptions including the one for "national security," I wonder how long it extends the time limit on actual government evidence of extraterrestrial UFO's.

/removes tinfoil hat

[indiecore]

I'd imagine that if there were any UFOs and they were classified it would probably be similar to the "born secret" clause that nuclear weapons research gets. So if you discovered anything it would retroactively be classified even if you didn't know it at the time and you could be prosecuted (and worse) for sharing it.

/removes Faraday suit (Tinfoil hats aren't effective anymore)