Hmmm, very interesting. I have very similar looking leaf troubles, but not only on the lower leaves. One plant - Kimberley - is affected all over. The chlorotic spots in this case are very close to the main veins shown in pictures below. Some lower leaves are still looking fine, so it doesn't fit the profile for Mg deficiency (thank you for the helpful details, Ivan).
Last week I found a couple of very brightly striped lower leaves on another plant, like Raybo's picture, which I removed. Now I'm seeing some dappling of pale chlorotic spots on a number of the plants, mostly on a lower or middle leaf, not all around one level.
I've noticed in the past that light chlorotic spots would sometimes appear on the leaves during cold stress, but disappear again after better conditions returned. I wonder if cold can also inhibit uptake of magnesium or zinc (very nice photo of the zinc deficiency, Ami). This wouldn't explain Kimberley's condition, since it wasn't cold when it started.
Was also reading this morning about tomato chlorosis virus, which looks similar to the more extreme condition, spread by whiteflies.
http://www.agf.gov.bc.ca/cropprot/emergingviruses.htm
The leaf troubles appeared just after a week of emergency reno's on the greenhouse, when it was open to small pests and also spores from the area, in unseasonal warm and humid weather. The same thing or similar is affecting my pepper plants in the house, too, pretty severe chlorosis appearing almost overnight.
