Huh, Blogger was disappearing my comments rather than posting them (has anyone else had this problem?), so I've temporarily reverted to having a separate 'post a comment' page (which seems to be working more reliably -- though you might want to copy your comment to the clipboard before posting, just in case). It also has the benefit of letting you subscribe by email to follow-up comments.
Update: let's try the embedded comment form again -- they've now added a 'subscribe' link to this too (see bottom right). Please email me if you encounter any problems.