By free forum host I assume you mean.. provide free hosting for forums?
Do you know if there's a market? Do you know people who need free hosting for their forums? Perhaps people who would be willing to make the move?
Do you already have hosting costs covered or would you expect to just eat costs for a few months and hope some of your freely hosted forums took off?
As a value added service or a way to get your name out it could be a powerful tool.. It doesn't sound like a good way to get 'started' though. The key would be getting high profile forums to take you up.. then of course, bandwidth becomes an issue.
One thing I've learned about forums, though.. and especially to keep in mind if you host them.. make SURE you provide a daily backup service. That could be a small additional fee perhaps.. offer free weekly and daily backups for a fee (or perhaps twice daily?)... That's a huge deal for forums

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Keep up the good work!
-Jonathan