Since Yudkowsky's 30-day Karma is currently 6055, and he hasn't posted that much over the last month, the answer must be "all votes on all posts within the past 30 days, regardless of the age of the post or comment".
This doesn't look that useful, I think it should only count votes on recent posts/comments. Also, this measure, if amended this way, looks informative enough to allow an option of viewing a longer list of active contributors, which could be linked to from caption of the sidebar section, like with "recent posts".
Even though I'm the one who originally suggested weighting top-post karma more than comment karma, I've felt that 10 karma per vote for a top-level is a bit too much. Five karma would seem more appropriate, weighting top-level posts more but also giving a more realistic option to climb in ratings via comment karma alone.
I wonder if a "top 100 contributors" page, with both all-time and 30 day options, would be a good or bad idea. It might more effectively encourage participation, but also make people more addicted and possibly cause undesirable status dynamics.
However "all votes on all posts within the past 30 days, regardless of the age of the post or comment" would make it easier to realize when someone has been abusing the system for retaliation.
It doesn't just count upvotes, as roland currently has negative Karma delta. You were probably downvoted more than 30 days ago (on comments that were since upvoted to positive total), and these downvotes are not counted in the 30-days sum, but count in the total. But this does look suspicious.
Right under the regular total user karma indicator.
Also for people who don't have a science background and might not understand the question, an upper case delta ( Δ ) can be used to indicate change of a quantity. So delta karma is change of karma (in this case change of karma for posts made in the last 30 days).
Is the karma shown that accumulated from posts and comments that were made within the past 30 days, or from all votes on all posts within the past 30 days, regardless of the age of the post or comment?