It's all about them not using a visitor ID.. if they use that it won't get spidered very well or at all.. best ones are the ones that make static HTML pages from posts.
In my experience (with IPB, PHPBB and vBulletin) - I run forums with all 3, vBulletin usually gets spidered best, due to you not needing to do any external modding really, e.g. sid mod for phpbb.
Also, after those mods it's generally better, but as said before, do the html post mod to get the best benefits.
Linear Mode
