It seems that Google has pushed many forum pages into their Supplemental Index. There are some reasons for this move that make sense.

Forum content:

1) Tends to be low quality
2) Tends to be repetitive
3) Doesn’t tend to have many deep links direct to it

The trend has been going on for some time. I’ve observed that many Vbulletins, in particular, have seen many pages erode. I guess it’s inevitable in a way, and it’s giving rise to an idea I have to keep the “sludge” to a minimum.

If Google is ignoring new pages on a basis of “quality”, it will be harder and harder to get many pages indexed without the help of many incoming links.