You should notThat is not how you change the filetype association. If you do that you have to manually rebuild(In reply to comment #0)
> Version:           unspecified (using KDE 4.4.4) 
> OS:                Linux
> 
> I've set KDE to prefer the webkit kpart by editing the desktop file (set
> InitialPreference higher than khtml's)
>
> When I open links by clicking on them in some application (tested: kmail.  It
> always happens), they are loaded with khtml.  entering the same URL by hand, or
> opening it via a link on another web page, or opening it via the history
> autocompletion, webkit is used.
> 
> I mostly don't notice this, but since khtml has a rendering issue with doodle,
> and since doodle URLs are mostly sent around by email, I am occasionally
> reminded of this.

Use the proper tool, File Associations entry in Konqueror's config dialog, to change the preferred part for handling html. Otherwise, you have to rebuild syscoca yourself (kbuildsycoca4) or restart KDE completely...

I *did* execute kbuildsycoca4, and the problem persists also over new KDE log-out/log-in.

The reason why I'm modifying the desktop file:
 * there is no good UI to say that I want the webkit kpart preferred for all MIME types that it registers (not just text/html)
 * I want the kpart to be preferred auomatically if the Debian package is installed on a system (the modification is part of the Debian package.  Yes, I'm the packager...)

On Tuesday, June 22, 2010 04:28:39 Adrian von Bidder wrote:
> https://bugs.kde.org/show_bug.cgi?id=242418
> 
> 
> Adrian von Bidder <avbidder+kde@fortytwo.ch> changed:
> 
>            What    |Removed                     |Added
> ---------------------------------------------------------------------------
> - Status|RESOLVED                    |UNCONFIRMED
>          Resolution|INVALID                     |
> 
> 
> 
> 
> --- Comment #2 from Adrian von Bidder <avbidder+kde fortytwo ch> 
> 2010-06-22 10:28:38 --- 
> I *did* execute kbuildsycoca4, and the problem persists also over new KDE
> log-out/log-in.
>
> The reason why I'm modifying the desktop file:
>  * there is no good UI to say that I want the webkit kpart preferred for
> all MIME types that it registers (not just text/html)
>  * I want the kpart to be preferred auomatically if the Debian package is
> installed on a system (the modification is part of the Debian package. 
> Yes, I'm the packager...)

Here is a simple procedure you can follow to make kwebkitpart the default 
rendering engine globally in KDE:

#1.) ALT+F2 and type 'filetypes' minus the single quotes.

#2.) Change the Embeded part for "html" and "xml" to KWebKitPart.

#3) Copy the file generated by the "File Associations" dialog above, 
mimetypes.list, from your personal "xdgdata-apps" folder to the global one. 
You can find the local and global "xdgdata-apps" folders by issuing the 
following command:

kde4-config --path xdgdata-apps

At any rate, this issue is not a kwebkitpart bug at all. The fact that a 
System Settings or a Konqueror specific option for selecting the default 
rendering engine is missing is not a kwebkitpart fault.

Regards,
Dawit A.

As stated in the above comment, comment #3, this issue has nothing to do with kwebkitpart which itself is simply a component, but rather the embedding application which in this case is Konqueror. 

Moving this report to Konqueror's queue. Perhaps someone else might be able to shade better light on it for you...

This is no longer a valid issue since Konqueror now offers the ability to change the default rendering engine to KWebKitPart. Changing that setting instead of messing around in .desktop files is the recommended method of switching the default rendering engine.