Hi, I spoke with my dermatologist about it and he said the idea to avoid sun exposure is basically because of the immune supressant action of the drug, which increases the ability of cancers to form and grow. It is nothing magic about the drug and sunlight, it is just that with the immune response suppressed in your skin there is a bit more danger of cancer formation, and the sunlight is linked to skin cancer formation [for the exact same reason, because the UV acts as an immune suppressant] so that is why they say you should not go out in the sun and use protopic at the same time. In theory, the same thing applies with topical cortizones or any other immune suppressant, hence when you take prednisone for example you are supposed to avoid getting too much sun as well.
That is how I understand it.
My own personal feeling is that the risk is way overblown. Doctors have caused so much more health problems by creating epidemic vitamin D deficiency [from constant sunscreen and avoiding the sun] than they have ever helped in terms of cancer prevention. It is absolutely ridiculous. I'm pretty blase about skin cancer, if you get unlucky it will kill you, but 95% of the time they just cut it off and it's no big deal, even with melanoma it is only fatal 25% of the time, and the medical link between melanoma and uv radiation is extremely tenuous, with more and more dermatologists raising a dissenting voice to say that uv radiation has nothing to do with melanoma.
Anyways... Personally, if I was using protopic a couple times a fortnight, I wouldn't even think about it. That really isn't very much at all. But if you are planning to go sunbathing a lot or risk getting sunburned then I wouldn't do that and use protopic at the same time. I think the immune suppressant effect of protopic is pretty localized, and therefore the skin you are not using it on would not share the increased risk from sunlight.
My two cents.