Initially I thought it was possibly my work machine playing up when the Visual Studio 2008 HTML designer was crawling. I went through and applied all the usual tweaks for 2005 in an attempt to speed things up, but no joy.
So I'm quite relieved to find out that it's a known bug. A hotfix is due shortly, and Scott Guthrie has promised to blog about it when it's released.
Until then, it's back to Notepad for me! Woo!