Didn't see this discussed before.
What earns contribution points towards GISP certification?
Does volunteering to answer on forums help contribute?
This link provides the point schedule for the contributions sections of the GISP portfolio review.
I know contributing to the National Map provides contribution points (.05 pnts/ hour), but I have not read anything about forum participation as being counted. That may be a good question to email to email@example.com.
Update: Here is the most recent response from Bill Hodge, Director of GISCI.
"...that is an excellent question, and I do consider that to be a worthy activity. The problem is whether or not the forums have some way to track and rate activity. If they do have some ability to show activity, then this would certainly count!"