Applicable Configuration:
Operating System: Windows XP
Web Browser: Opera, Firefox

Descriptive Problem Summary:
When replying to comments, things are no longer separated by line properly. For example, assume the following is how it was supposed to look:

Bob wrote:

I say, good sir, "Bloop!" and I say such a thing heartily.

Alas, you have lost your marbles!

This is how it showed up now:
Bob wrote: > I say, good sir, "Bloop!" and I say such a thing heartily. Alas, you have lost your marbles!

It can be seen here several times.

[Edit]
This is apparently more than a problem with posting, as this is happening to all blog comments.

[Edit 2]
In fact, all newlines are being converted into spaces.

Numbered Steps to Reproduce Problem:
1) Reply to a comment.
2) Post.
3) View.

Expected Results:
The text will be formatted with proper linebreaks.

Actual Results:
Linebreaks seem to become spaces

Does the problem occur:
Every time? Or how often? Every time.
On other computers? Aye.
In other user accounts? Aye.

When does the problem NOT occur?
It always seems to.

Workarounds:
None, as the <br> tag is filtered out.