Big gap b/w first and second entries on a page

In the last week or two, a large gap has appeared between the first and second posts on my Friends' and Recent Entries pages. I'm running Windows XP, and it happens in both Firefox 1.5 and IE 6. Anyone else notice this? Any ideas what I can do to fix it?

EDIT: I see it on the s2_tranquility page too! The second post on the page seems to start where the sidebar ends in both cases.