Well, I just restarted my computer after 26 days of uptime. The impetus was that my startup drive finally got down to less than 100 MiB free. I've noted before that Mac OS X seems to work perfectly fine under suboptimal conditions where you have only a tiny bit of free space on your startup disk. (It was at 0 KiB free for about 30 minutes or so, but I rectified that by deleting some apps.) I had been running at below 300 MiB free for a couple days now, too. Not because of any crazy stunt, but because I was lazy and didn't want to restart my computer.

Well I finally did today. My free space shot up from 100-or-so MiB to 2.35 GiB!!! WTF was taking up all that space?

Besides a general slowdown which occurs even when you have enough free space, there was only one other odd thing that I noticed: when you have no free space left on your startup disk, your iDisk refuses to mount. You can press Command-Shift-I all you want, but the menu item just highlights and does nothing. Bug? :)