Yes, forums can be very profitable. I know a few people who make tons of money of forums. However, don't rely on ads too much. Try making exclusive type of membership, such as VIP members who'd have to pay a little sum to be in. That, and donations should be the main source of income. Also, once you make a successful forum, it's easy for you to promote your future web projects with it, as members pretty much worship the admin.
And what about this forum? Do you think that forums.digitalpoint.com owner was really earning $10k per month with adsense? It's really hard to believe how digital point is ranked 180 three month alexa ranking...
this could easily go few thousand bucks for a good, secured server. then you really have to have somebody full time working on monitoring the forum, i dont think unpaid mods in most cases would be enough.
If you're spending that much on your server then you must have a decent amount of traffic - if you can't profit from that traffic then you're doomed to fail.
The more specific the topic and ads the better the chance for clickthrough. Some broad general sites work to by shear volume and some luck, but focus on specifics and a tighter community and you should see a good site.
Yes, they are for longterm as well but a lot of hassle to handle but worth having longterm user base. If you are looking for quick money than stay away...as they take much time...
As hard as it is to set up a real good forum in a niche that is popular, with time and effort you could have a real little gold mine on your hands. For me, adult forums have always been the best money maker.
There's no short cut to success...you have to give it time and effort, no cutting corners if you want to succeed.