I've repatriated but I lived there for 2.5 years on student visas. You only have to attend classes a few days a week and, it's Thailand, so a passing grade means showing up. My first year I went 3 days a week for 2 hours a session. My second year I went 2 days a week for 3 hours a session.
It can seem like a pain in the ass but it's not that distracting. In fact, it can be a nice break and get you out of your apartment for a few hours a week. Plus you get to learn a little Thai. Or, some Muay Thai schools also offer an educational visa package if you're more physically inclined.
And, I never had a test so I don't even know what the reference to "passing grades" was about. Most of the people in the classes are only there for the visa too so you really don't have to worry about being shown up by other students. In fact, most of the big schools (Pro Language, Walen, etc) almost seem designed to help people get visas. Learning Thai is an afterthought.
Unlike most of the other visas, you do not need to leave Thailand every 90 days. You have to leave Thailand to get your initial visa but after that you go to the immigration office in Chang Wattana (if you're in BKK) every 90 days and apply for an extension (which is almost always granted without question).
After 2 years of classes I am not even close to fluent but can get by in Thai. I can also read and write in Thai (though I don't always understand everything I read due to not having enough vocabulary). Being able to read and write in Thai is a nice little party trick. The Thais can't believe it and are always amazed
Nicky and clyde recommended Sing and Malaysia for visa runs. Even when I was doing border runs, I never did either. I've done visa runs to Hong Kong, Laos, Vietnam, and Cambodia. I think Laos, Vietnam, and Cambodia are your best bet for back to backs (though Cambodia runs hot and cold at times). Sing is really cracking down and I know people who have been refused back to back tourist visas. People going to Malaysia have also reported some hassles getting back to back tourist visas.
Like geomark said, don't set up a business to get a visa. It doesn't work like that. You have to be running an actual business and hire Thai employees. Setup costs will run you about $1000 with attorney fees. Plus you have to have a physical business location. And they have all sorts of crazy requirements like making you send in photos of you and your staff or the building you're doing business in. Way too much of a hassle unless you actually want to have a Thai registered company.
If you need any contacts or recommendations on Bangkok . . . PM me. I own a social networking site focused on Thailand and expats so I know tons of people there.