The issue of GPL only comes into play at two points. The first is people who want to load the Worldkit SWF into another SWF project. The second is people who need to tweak Worldkit's behavior at the code level.
On the first issue, the GPL does not permit loading Worldkit SWF into another non-GPL'ed SWF. "The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License." LGPL or BSD licensing gets around these issues.
As for the second issue of modifying a GPL-ed Worldkit, all is fine except that I'd have to re-publish my code modifications. This is largely a non-issue for people tweaking the API, and most likely a good thing. (By the way, the GPL item 6 requires you to clearly link to the source from a "non-source" form. I'm not sure you're 100% GPL-compliant on that point. I'll post to the osflash mailing list and see if I can find the popular approach to this.)
Basically, the GPL likely hinders people from using Worldkit as a Flash component. If the Worldkit API provides adequate public methods, the LGPL would be a nice compromise. BSD, of course, would work too.
