Good luck with this change. Almost every time I've done a search on Google the past year for anything webmaster related, some WW thread come up on the first page. I know I would have never found this place without the first Google search. Probably the best way to judge if this new robots.txt deal is hurting or not would be to keep tabs on the average number of new forum members that join in the next few months and compare it to the same months last year. If you see a drop you might have made a mistake if you want to continue to draw new folks to the forum.