I've now also lost the ability to submit to my original thread, exact same problem. Following up on Matt's question, the problem repro-ed on Firefox 3.0.2 and IE7, as mentioned in the OP. Sheng repro-ed on Chrome. If you don't see this yourself, please consider our moderator status. The weirdo text that appears in the Post Option box that push the Submit button out of reach look like hints for moderator options. Please send me an email if you need my password to login as me and see it for yourself.
Reposting what I see in the Post Options box:
Post Options
This
option will split a single thread into a new thread at the specified
post. Please include a subject for the new thread, a reason for the
split and choose the forum where the new thread should be created.
Note: any posts split will lose Proposed Answer or Answered state.
This
option will merge the currently selected thread into a target thread.
Note: Answers and Proposed Answers will be removed after moving and
this action cannot be undone. Please enter the thread ID to merge to.
The thread ID is the portion of the URL after /thread/ which looks
something like this: 8138b571-18b6-475a-85d0-63423e506875.
This
option allows the user to change the thread type. Note: changing the
thread type will remove any Answers or Proposed Answers from question
threads.
Submit button out of reach, part 2
icon to add code.