This is a question I get all the time and something you should know about before you buy an older home. As a disclaimer, I'm not an electrician, but I've been on tons of home inspections and spoken with many electricians on this topic and this is what I've learned.
Knob & tube wiring is an older generation of wiring that does not have a ground wire. Typically its found in homes 50 years or older if the electrical system has not been updated. In Haveford Township and most of the Philadelphia Region, Knob and Tube is still a very common place and should be expected. If you have an unfinished basement or open attic space, you could identify this type of wiring by the cloth sheathing around the wire and the white porcelain knobs that hold it in place.
Is it dangerous?
It can be, but its not the end of the world. I would definitely recommend having a qualified inspector examine a knob and tube system. Many qualified electricians and inspectors have told me different variations, but the same overall message. Because the system is not grounded the wiring can overheat especially when hooked up to devices that draw more power (TVs, computers, electronics, etc.). In the case where you may have a bed lamp or a ceiling light hooked up to a knob and tube wire, most electricians don't see this as a safety issue. The problems and issues occur when someone has attempted to add onto a knob and tub system and/or begun to splice new wiring with old. Other instances are when you find homes where insulation has been added and is against the wiring. This can be a fire hazard. Knob and tube wiring needs to have a air space to function safely.
Can you get home owners insurance when there is knob and tube in a house?
Yes, many insurance companies, big and small will insure a home with knob and tube wiring.
Should you replace all knob and tube wiring?
I recommend having a licensed electrician take a look at the system. If its in bad shape, replacing it makes a lot of sense. I know several home owners who've taken the approach of replacing knob & tube wiring where ever it is found when they do renovations. Others have added grounded romex lines to the areas where they have greater needs for electricity. Rewiring an entire home can be expensive, so having some options is helpful.
Who should do the work?
I recommend finding a local seasoned & licensed electrician. By this I mean, someone who's worked on old homes and has experience in this type of work. They see these types of systems all the time and know what they are talking about. For electricians who typically work on new construction projects, they may cost you a lot more time and be quite a bit more expensive. There's definitely an art to pulling wires through a 80 year old home.
What should a home buyer do if they find knob and tube?
Have the situation evaluated and see what makes the most sense. Remember, what ever home you buy will not be perfect and this just might be one of those issues you run into.
Any further questions, let me know. If I don't have the answer, I'll put you in touch with someone who does.