I think I made a similar suggestion a while ago. Filtering cancelled and waitlisted activities - not deleting them, just having an persistable option to hide them - would be great. I'm in the basic class this year and I have already signed up for about 10 activities which I later either canceled (when I realized that being at position 8 on a waiting list wasn't going to get me anywhere) or where I remained waitlisted and no place had become available.
I agree with Brian that cancelling at the last minute when you are on the roster is an annoyance and a headache for the leader, but I'm not sure if merely the number of canceled activities would be a good indicator of unreliability. I removed myself off the waiting list a few times but not from a committed roster. Should that distinction be available to trip leaders?

To Brian Booth: I believe the idea is to add an option to hide/display cancelled activities rather than just delete them.

Jeff, I think in order to deal with large lists, a time period filter would be effective like when you look at the bank statement online.

At first, I was about to agree. But then I realized there IS value in displaying Canceled activities. I am a Scrambling trip leader. I have one individual who has signed up for 2 of my trips, then cancelled at the last minute, causing a hassle for me to re-print my trip roster for my emergency-call person. Just recently I checked this individual's Activity History, and learned that he has signed up, then canceled on 18 activities in the last 2 years! If he signs up on one of my scrambles again, I am going to be very firm with him and say it better be for real this time - please don't sign up unless you really intend to go. Please keep this; it's a valuable tool for Leaders.