Not sure about this.. but the mark "KR-85" on most modern metal-halide lamps always confused me. At first I thought this was a coating. Now I know it's a gas filling but didn't understand why it was stamped on the lamp. I learned it's a radioactive isotope of Krypton and I think it's used to "make the lamps easy to ignite". But if the half-life of Krypton-85 is 10.756 years, then storing a lamp more than 15-20 years would ne impossible or very hard to ignite? http://en.wikipedia.org/wiki/Kr-85

The lamp design count on that, the ignition suffice with just fraction of the original fill amount, so even after few decades it will still be effective. In fact you need just one nuclei to break for an sucessful ignition, so after the half life the rate would be just half, so it will take just double the time required for ignition, so instead of one second it would be two (assume just the Kr decay as the only effect), so no big deal.
Yes, after very long storage the lamp would be harder to ignite. But not only because of the Kr85 decay.
But there are many other factors causing the lamps do not last infinitely at storage: Corrosion (mercury into other metal parts,...), cements/plastic degradation, gas cleanup/contamination (that would be probably the worst, although mostly recoverable to a big extend)...

No lamp is designed to allow so long term storage, even the ~20000hour rated lifetime is supposed to be used within about the 10years upon manufacture, they do not expect the lamps to be stored for so long before use.
Maybe some military versions would have such specification, but even there that would be rather an exception.

Thanks Medved for the clarification. And the Kr-85 stamp is there as a warning of some kind I suppose.

So I need to know, what you're saying is, it is better to run a lamp once in a while? I need to know for metal-halide and mercury lamps..no matter if it's a rare lamp or one collected for posterity (museum piece). I suppose it's all in the collector - Am I interested in the spectrum & output or the construction of a lamp? For myself it's the spectrum/color/lumen/CRI.. it's best to run the lamp a little to make it last at least it's rated lifetime?

It is very individual. Some degrade just as they are sitting, some during warmup, all lamps do not like starting cycles (with many of them the life is in factt limited by the number of starts, rating is made for 10hours/start, one start/day)

Some designs suffer just after startup, so once you ignite them, they have to warm up and stabilize and restore the chemistry fully before being switched OFF (LPS penning mixture restoration needs all the lamp to heat up, so release back the trapped gas; MH's need to complete the halogen cycle clean up of the sputtered tungsten,...).
Storing the lamps for long time would always be a gamble: Either they start after the years, or not. Or they may fail during warmup.

So once you decide to ignite the lamp, always keep it running for at least 30 minutes. And when doing so, always treat the lamp like it is going to disintegrate (explode,...), so cover it into something robust enough to contain the hot pieces should that happen. Even when the lamp is an open rated type (include HPS, MH, LPS), you can never be sure, what faults developed over the long time or the sudden stress after that long time, the safety features may not work correctly anymore.

And if oerating them as a preventive measure, I'm not sure. With some it may help, but with many you will just give them an extra stress cycle, so they will be more likely to fail...

Thanks for the info, understood