The Harvard sign has been very noticably off since last week. I can't recall positively seeing it lit since before the first of the month, so I suspect that its been turned off due to the end of a contract period.
Its always been on a speed sensor of some type (probably radar -- I don't think a simple radar speed sensor is that expensive anymore). You could usually tell if the ad was going to light or not -- if the train, leaving Harvard, got any restricting signals bewteen the station and the crossovers, it usually wouldn't be up to a high enough speed for it to come on. In my commutes, I'd say about one trip in 10 for me it wouldn't light up.
The Target ad also seemed to be up for way too long -- even occasional riders had seen the thing many times over by now. I wonder if they're having trouble finding new clients, as I would think it would have been changed out very quickly if they had another ad ready. Perhaps its not taking off with advertisers as much as they had hoped (kind of like how the wrapped buses were a fad at first, but now they seem to have disappeared -- the only wraps I've seen lately are on the Green Line).