But I managed to find some actual research on this and the answer is no. Or at least in such small amounts as to be completely negligible.
No vitamin D (ergocalciferol or cholecalciferol) was found in the basal diet. The 25-OHD concentration in the plasma of kittens exposed to sunlight declined at a similar rate and attained the same end values as kittens kept inside a building without exposure to UV light (Fig. 1).This observation suggested that cats do not synthesize adequate amounts of vitamin D from exposure to UV light to maintain plasma concentration of 25-OHD. As the rate of decline in plasma 25-OHD was not slower in the kitten with the hair shaved than those not shaved, it suggested that limited exposure of skin to UV light was not the primary reason for the lack of effective synthesis of vitamin D. One kitten from the exposed group developed posterior paralysis that was subsequently found to be associated with vitamin D deficiency in cats (Morris et al. 1994).[source]Given a diet lacking in vit D, even a shaved kitten exposed to sunlight had it's blood concentration decline at the same rate as the unexposed control kittys. The study doesn't mention whether or not it got sunburn.
Still, it is interesting how much cats love their sunlight. And it doesn't seem to be just warmth as they will do it all summer long (indoor cats at least). Perhaps there's another reason, such as the sunlight helps to soften oils that are spread by grooming.
Of course, they also loved to lay on televisions and monitors back before those all got too thin for the job, so it could mean nothing at all.