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".
This would be so much more useful if it were based more heavily on comments. Right now, it's just "people who made a popular top-level post in the last month", which is information you can easily find by just scanning the front page.
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?