Blogger support confirms that there is a known bug with the BlogThis! bookmarklet, and that they are working on solveing it.
The bug: When you select text from a webpage and click BlogThis!
the script often inserts the selected text in the url field, and sometimes it inserts nothing at all, only the title.
the bug seem to be the same om Firefox and IE at least (windows platform).
The temporary solution: Log in to blogger first, through blogger.com, and then use the bookmarklet. Not a very cool solution, but it’s good to know of it, while we wait for the bug to be fixed.