Now that fios has their faster residential speeds, they're plenty fast to host a start-up forum. I just upgraded my fios to 75/35.. but they also have 150/65 and 300/65. For hosting something on your residential line, you're looking at the 2nd number - that'd be your users' download speeds.
You could grab a pc you're not using (I grabbed a dell r200 from work) and set it up with a Linux distribution.. set it up like you would if it were a dedicated server at a datacenter.
You'll want to forward port 80 through your fios router - and probably port 22 as well so you can get to it from the outside.
Also, don't forget to set up good firewall software - csf/lfd is free and is a great solution. I'm blocking 5-6 IPs/day for port scanning and failed logins on my home network currently..
The only issue you'll run into is outgoing mail since verizon blocks port 25 from getting out. One workaround is to set up your server to send mail through a gmail account - which it could reach on port 587. All mail will appear to come from that gmail account, but users signing up to your site will at least get mail.
You'll also want to keep an eye on your IP address - verizon doesn't seem to update them very much - sometimes when the router reboots it will change though. You can use a service like dyndns to keep track of that.
Anyway, for a new forum, this makes a great way to grow it with free hosting. Once it gets a little popular or starts making money, then you can look into a good secure hosting solution.
Is anyone on here doing this already?