Yup, they're still photosynthesizing.
Remember, chloroplasts are not bound by membranes within the cell, and as such they're free to move around. When there's lots of light, chloroplasts will line up behind one another (like stacking, with the sunlight coming from above) so that only the the few blocking the extreme sunlight will get damaged (this's why plants in lots of sunshine are often a lighter green.) In dark conditions (eg shade,) chloroplasts will move and spread out, so that as many are getting light as possible (thus, plants which grow in lower light conditions are often a darker green; think forest floor in a forest with a dense canopy.)
While a a few days of reduced light due to clouds wont generally cause any major visible differences in leaf shade (IME,) they're compensating for the weather and making the best out of it.
An excellent example is Germany. Parents will sometimes need to get vitamin D supplements for their children because they aren't getting exposed to enough sunlight, so their bodies don't produce enough Vit D on their own (there are years when you get just a few days of sunshine.) Yet areas like these often have vast amounts of vegetation. Hamburg, for instance, looked at from above looks more like a forest than a city due to the large gardens and extensive greenery, when it fact it is a pretty large city.
There are definitly cases when things are better when they're cloudy. Certain plants will close their stomata when there's too much sun (otherwise they risk drying out,) this hinders gas exchange and if it occurs for an extended period of time, a plant will use O2 instead of CO2 in the photosynthetic process, this reaction ends up actually just wasting energy for the plant. A bit of cloud darkness isn't a problem, it's generally too much sun that you should be worried about! Furthermore, it's better to have a sun-adapted/acclimated plant in shade, than a shade-adapted/acclimated plant in sun.
To sum it all up: Photosynthesis is still going on, you gotta try REEEAL hard to get it to stop.