Multiple modules display <LF> problem on first line of RSS feed

I've already opened an issue for the captcha module here: http://drupal.org/node/36453

Now I've implemented FCKEditor since Xinha/HTMLArea IMO sucks like a black hole, and I've identified two other modules displaying exactly the same behavior when activated:

- FCKeditor
- weblink.module (I know, there's a replacement coming sometimes in the future)

And I'm almost sure there are more to come.

My current setup:

- apache 2.2.0
- php 5.1

But the problem did also happen on apache 2.0.54 and PHP 5.0.x, so I don't think problem lies withhin a minor version of PHP or Apache.

So it is probably some initialization routine from a module inserting that spurious linefeed into the RSS feed.

Anybody got a hint where the bug's hidden?