The better kind of forum for SEO is where: A- Your links are dofollow. (Obviously.) B- Your forum profile is visible to the search engines, and your posts have a dofollow link to that profile. B is important since your forum posts will channel internal PR to your forum profile. Not all vBulletin boards are setup like this. Some bulletin boards are setup so that the profile is only visible if you're logged in and the link is nofollowed. Without a public forum profile, you will just have hundreds of extremely low quality links. Hundreds of low quality links is worth less than the higher quality backlink from your profile. A way around this might be to have your signature link to one of your posts (or a post in which you've commented in; preferably relevant to your site). Or simply avoid posting on those forums as they have almost zero SEO benefit.
The important fact behind Forum Posting is Getting targeted traffic. The signatures or the website at your forum profile will help Search Engine Spiders to find your site easily(Only with big forums like DP)! Forum posting will also give lot of backlinks but they are worthless. Lets take you posted in 100 threads which are atleast PR2. This will not definately help you to increase your PR. SEs ignore those links!
Most (decent) forum posting companies will make sure their writers are using the keywords a client will request. Its something simple to say "use these keywords" rather than have someone randomly post on your site all Willy nilly like.
I agree with johnchow. SEs ignore backlink and don't helpful to get good page ranking but we can grab large traffics from forum like DP.
And we too could get results for our sites especially if we choose well the forum site that we used.. like DP as well.