I hate to actually post my whole idea here in case theres any developers who like my idea here but here goes. I want to use and re-code the phpBB2.0 core code but keep and secure the templating system of phpBB2.0 so I could release many styles for my forum hosting service. Are forum hosting service's successful?
You will have to come up with a creative "no money back guarantee" provision in your terms and conditions. Forums are hard to spin out, but there are still many enthusiasts out there who'd give it a try. Of course after about a month they will be demanding their money back since their forums will be as good as dead, so you'd have to have that provision in place.
Forum Hosting is a business of the late 90's when there wasn't a lot of GOOD forum options and web hosting was more expensive and complicated. Most users can launch a forum through their web host with a click of a button. And the cost of web hosting in some cases can be as low as $4/mo. I would strongly advise against it. I don't think you will make money. And even if you do sign up clients you will have to charge dirt cheap and the amount of money generated wouldn't be worth the up keep.
What kind of forum hosting service? If you're charging per package you could look into opting for a reseller package and then charge offering a special forum hosting service, a good niche as forums can grow. If you planned to do the more traditional, free forum hosting then depending on your script you can opt for a shared hosting package which costs from $3+ to start, as you grow, expand the hosting you're on. It can be a great source of revenue for you from advertising and paid upgrades. Good luck!