Anonymous guests posting isn't a good idea.
Malicious users can post and run off.
I was considering banning hotmail, yahoo, gmail and force members to register using their isp emails because of some repetitive abuse in forumand people always managing to re-register using a new email becauz they can get new ones infinitely.
If it was me, i'd make registering to forum as a pre-requisite to even be able to download Imho.