I install security systems for a living and completed a contract for a home along Cossart Road over the summer. It's not the property of the infamous cult house(s), but it's within a short walk through the woods to get to it and take a look. There are pictures and some videos available on the internet; most are about as high quality as you'd expect from a bunch of weekend ghost hunters with Motorola Razrs and the Best Buy special cameras, but they get the point across. You're not going to find anything taken at the front door because, simply put, if you get within more than a quarter of a mile of many of the estates in the area you're very likely to be apprehended by the security firm that patrols the grounds and turned over to the state and local police. You're not going to be put in prison, but the fines that come along with the trespassing charges are substantial.
From what I can tell after spending three months in the woods around Pennsbury Township, there are about four homes that could be considered "cult houses" depending on who's posting a story about it on any given night. One is a home approximately half a mile from the Route 100 intersection, and is the only obvious set of gates right along the road. This house doesn't exactly qualify as a mansion, and while gorgeous is somewhat modern compared to the other homes in the area.
The second is located about a quarter of a mile northwest of the gated property mentioned before. It's actually not connected to Cossart at all, but has a driveway coming off of Fairville Road just to the north. You can't see it from Fairville. There's a rather large wall of trees planted to keep it from view, though it is clearly visible from the peak of the hill traversed by the driveway to the first home. This is probably the jackpot for most people reading this thread. The house does have some fairly gothic looking window panes which resemble crosses, and while these are definitely on the eccentric side of architectural design the look is not especially uncommon throughout Chester County, and in old construction in general.
The other two possible homes are probably the most difficult to get a glance at. They're both right around the midway point of Cossart's stretch between Fairville Road and Creek Road. The driveways aren't especially long, and one is gated, but for the most part their visibility is severely obstructed by the surrounding woods. Neither one is any more creepy than the rest of the homes in the area, but most of this stems from knowledge of the legends and the general appearance of the township.
There is a private security group which works on the grounds. Their employment is a joint effort between Pennsbury Township and the wealthier homeowners along Cossart. They are equipped with a variety of SUVs and pick-up trucks, and have tracked and monitored vehicles that shouldn't be there. Regardless, a huge number of the chase stories and experiences are just teenagers screwing with each other.
I can answer more specific questions if anyone cares, I guess. I've met the owner of the infamous Cult House and he's actually a fairly mild-mannered, nice guy. He's just grown sick of the intrusions and vandalism, and from what I've gathered doesn't believe for a second that he owes an explanation to the public, if only because their constant destructive behavior has gotten to him and everyone else living there.
Like I said, any specific questions feel free to post them here or PM me. I suppose it's also worth mentioning that the Skull Tree is gone. It was taken out by PECO's subcontractors in June or July of 2010. We had a storm in early June which caused quite a lot of damage, and the tree was removed because it had become unsafe. There's still a tree that resembles it, but it's not the same one.